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The Indonesian Throughflow



As you head out from the shores of east Bali on your morning dive you are surrounded by a host of tiny outrigger fishing boats, their sails flashing in the sunlight as they dance over the waves. It is a unique, entrancing and inspiring sight, all the more so because this is a very special place.

These fishermen (and you in your dive boat) are scudding over waters that lie on the cusp of geological and biological change. In the past, at times of low sea levels, this coastline marked the very tip of a massive Eurasian Continent. During those periods you could have walked all the way from Bali to Paris without getting your feet wet. However, even when the world’s oceans were at their lowest ebb, you could never have gone any further east without harnessing wind or mechanical power, due to the depth of the chasm between Bali and Lombok, the next island in the chain.

A Fisherman contemplates the ocean

A Fisherman contemplates the ocean

A juvenile lion fish finds sanctuary in the mouth of a bottle placed in the sand in Amed East Bali

A juvenile lion fish finds sanctuary in the mouth of a bottle placed in the sand in Amed East Bali

Wallace’s Genius

In the 19th century the great naturalist Alfred Russel Wallace spent time in the islands of what is now Indonesia and remarked on how different the species of bird and animal living in the west of the archipelago were from those that inhabited the east. His findings led him to draw a line on a chart that ran through the Makassar Straits between Borneo and Sulawesi, crossed the Banda Sea and continued down through the straits between Bali and Lombok. To the west of the line, in Bali, Borneo and beyond, animal and bird life was predominantly Asian. On the other side of the line, in Lombok, Sulawesi and further east, the animals and birds were strikingly different, mainly species that originated in or resembled those found in New Guinea or Australia. Brilliantly, he deduced that the reason was geological rather than biological: “I believe the western part to be a separated portion of continental Asia, the eastern the fragmentary prolongation of a former Pacific continent.” Of course, we now know that he was absolutely right!

During his trip Wallace also came up with the notion that natural selection was the driving force behind evolution. At the same time Charles Darwin was on the Beagle sailing the seas on the other side of the Pacific and drawing similar conclusions. But that is an entirely different story.

A Very, Very Big River

What Wallace could not know, in fact something that no-one noted until almost 100 years later, was that one of the factors preventing species crossing Wallace’s Line was an enormous moving body of water. The fossil and historical records show that elephants, orang-utans, bears, tigers and other Asian mammals never managed to bridge the gap. They got as far as Borneo and Bali but no further. Even those animals that tried to swim over or were caught on driftwood after a flood could never make the crossing as everything that enters the water on one side of the line is swept away far from land long before it can reach the other side.

In the Pacific Ocean to the northeast of the Indonesian archipelago, the sea level is about 20 centimetres above average. In the Indian Ocean south of Indonesia the sea level is 10 centimetres below average. This 30-centimetre drop between the oceans creates water movement on a massive scale, a phenomenon variously known as the Indonesian Throughflow or Pacific – Indian Throughflow. The most direct path between the oceans and the path taken by a large portion of the Throughflow, as you may have guessed, runs right along the Wallace Line between Borneo and Sulawesi then down through the straits between Bali and Lombok.

Visualise it as a very, very big river pouring through a canyon between continents. How big? Ocean current flows are measured in units called Sverdrups with one Sverdrup being 1 million cubic metres of water per second moving past a given point. The flow of the Amazon is about half a Sverdrup and the combined flow of all the rivers in the world is about one Sverdrup.  The flow of the section of the Indonesian Throughflow that courses past the eastern tip of Bali is estimated to be 2.6 Sverdrups, that is, two and a half times the total of all the rivers of the world put together!

Mola-mola and Diver

Mola-mola and Diver


Creatures Great and Small

Thus, a vast amount of water is drawn from the Pacific Ocean and sweeps through tropical seas, across the equator and past the islands of northern Indonesia. As it travels, it picks up and carries along with it an enormous quantity of marine larvae, eggs and juveniles and deposits them wherever it encounters land. It is no surprise that most of the legendary Indonesian scuba diving destinations lie along the route taken by the Throughflow.  For example, the calm bays of black volcanic sand in northeast Bali are astonishingly rich in rare species. Enterprising dive operators need only sink a stripped motor bike frame or a dozen broken bottles set in a block of concrete and they soon become a collecting point for small fry seeking sanctuary. These tiny fish attract predators such as frogfish, lionfish, scorpion fish, eels and other rare and fascinating creatures and they in turn attract scuba divers and snorkelers.

Dive guides who work in the area often report larger sightings too, such as dugongs and a host of cetaceans, including killer whales. There are sightings of bizarre and highly unusual marine life. Last year an amateur diver captured video of a long entirely transparent eel; the film became a Youtube sensation.

A Plan of the Throughflow

A Plan of the Throughflow

Diving the Throughflow

Beyond the calm water in the bays, the Throughflow provides dramatic drift diving along unusual seascapes. Towering barrel sponges lean at 45 degrees as if in the presence of a powerful wind and, in places, pinnacles and walls are scoured of their usual natural coating. But in the valleys and other places where the topography provides a little shelter from the onslaught of the current, the enormous profusion of corals and fish life can take your breath away. In one bay, for example, just a few metres off shore from the lines of colourful fishing boats that decorate the beach, you can find hectares upon hectares of multi-coloured staghorn coral, as glorious a sight as any of Bali’s rice field terraces. Drifting over these fields of pointed sculpture in pastel shades, populated by clouds of damselfish which rise and fall as you pass is a true magic carpet ride.

Mola-mola in sun

Mola-mola in sun

Cool Animals in Cool Water

The Throughflow passes by the southeast coast of Bali too and close to the village of Candidasa a series of small islands, some not much more than jagged shards, are home to some of Bali’s fishiest diving. Schools of jacks, rainbow runners and barracuda use the dramatic rock formations for shelter from the current and reef sharks circle around watching for weakness and waiting to strike.

Further south, attempting to block the southern end of the Lombok Straits like an ill-fitting plug is Nusa Penida, the exposed part of an undersea ridge that connects Bali with Lombok. The Throughflow races by on both sides of this island, offering healthy coral, lots of big animal sightings and exhilarating drifts. Sometimes, however, the ride can be a little too wild and conditions can change rapidly and vary dramatically. One day you will drop in on a site and be entertained by a dozen manta rays, which sweep in from the deep to feed on plankton in calm, bottle green water. On another day, the same site will be empty of fish and the ocean will merely throw you around in a soup of spume and spit you out into the southern ocean on a fast train ride to the horizon.

Current pools created by the Throughflow off the coast of the village of Aas on the very tip of Asia

Current pools created by the Throughflow off the coast of the village of Aas on the very tip of Asia

Seasons have an effect on the speed of the Throughflow, with August usually the strongest month when the southeast monsoon is at its zenith. This also coincides with mola-mola season around Nusa Penida. The arrival of these bizarre creatures may be linked to the increased strength of the current but this time of year also sees an upwelling of colder water in the south of the Lombok Straits and that may be significant too. The reliability of mola-mola sightings during this period has created a small boom in the local diving industry. Be warned if you are thinking of joining the crowds: water temperatures can drop to the high teens centigrade but when you are face to face with an oceanic sunfish measuring four metres from tip to tail you will probably not even notice!

Simon Pridmore is the author of scuba diving books, travel books and, as you might expect, scuba diving travel books. Originally from the UK, Simon has lived in Asia for over 30 years. As well as his books and guides, Simon writes regular columns for a number of magazines. He and his wife Sofie currently live in Bali, Indonesia but spend a lot of time exploring other places trying (but failing so far) to find a cure for their itchy feet. Simon's latest book - Scuba Exceptional – Become the Best Diver You Can Be - the follow-up to his best-selling Scuba Confidential, is available now in paperback and e-book versions from Amazon stores worldwide. Find out more about Simon and his books on


Diving with… Ana and Miguel, Siladen Resort & Spa, Indonesia



In this ongoing series, we speak to the people who run dive centres, resorts and liveaboards from around the world about their businesses and the diving they have to offer…

What is your name?

Ana and Miguel

What is the name of your business?

Siladen Resort & Spa

What is your role within the business?

Co owners and General Managers

How long has the business operated for?

Almost 20 years.

How long have you dived for, and what qualification are you?

Miguel has dived since 1993 and Ana has dived since 2002. We are both Open Water Scuba Instructors.

What is your favorite type of diving?

We love all dive types, especially wall diving for the wonderful corals and looking out into the blue wondering what other animals will come up next. But as an UW photographer and videographer we both love critter hunting in the sandy slopes. We are both very fond of night dives as we always find interesting animals and different behaviour.

If you could tell people one thing about your business (or maybe more!) to make them want to visit you what would it be?

We have a wonderful location in the heart of Bunaken National Park and we have an amazing team that provides the best service to each guest. We are a fun dive resort; we constantly strive to be as sustainable as possible and safety is our number one priority. If you like sea turtles and beautiful coral reefs you cannot miss this area for diving or snorkelling.

What is your favorite dive in your location and why?

If we have to choose one wall we would say Mandolin as it has beautiful and healthy corals with a wonderful and shallow reef top. We love to search for turtles in the overhangs, look for long nose hawkfish in the black coral bushes, admire the schooling snappers, and spend a long safety stop on the coral gardens on the top that are full of anthias. For critter hunting and night dives we both love Bolung. Here we can find frogfish, ghostpipefishes and nudibranchs, plus at night we often find decorator crabs, octopus and even stargazers on its white sandy slope.

What types of diving are available in your location?

There are very shallow coral reefs surrounding all five islands within Bunaken National Parkpark — beyond the shallow reef, the seabed drops away very quickly, forming the beautiful walls that Bunaken is famed for. Many of these walls are vertical with huge caverns and overhangs, however some are more gentle slopes that allow more reef building corals to form. On the North Sulawesi mainland there are black sand sites that allow amazing muck diving. Close by, we also have white sand sites, which offer some beautiful coral reefs and gentle slopes. There is even a fairly intact (entirely sponge encrusted) wreck within easy access of the resort.

What do you find most rewarding about your current role?

Working together in a dive resort that welcomes divers and snorkellers from around the world and sharing with them our passion for diving, snorkeling and uw photography and video is a great pleasure. Besides we have the privilege to dive and snorkel at world class sites every week.

What is your favorite underwater creature?

We both love cephalopods in general for their intelligence, capacity to camouflage, change colors and patterns…. We are fortunate enough to have many encounters with cephalopods in this area, from the tiny bobtail squid to the wonderful broadclub cuttlefish, the flamboyant cuttlefish, squids and Ana even filmed reef octopus mating. We could watch cephalopods for ever. And although we see them often, Ana has a soft spot for turtles.

As a center what is the biggest problem you face at the moment?

The biggest problem we all face as divers and nature lovers is climate change and pollution. We need to work on these issues locally but even more importantly we need to address these issues worldwide.

Is your center involved in any environmental work?

Yes, we clean our beach daily and we organise very frequent island clean-ups. Not only do we clean this area, but we also separate the garbage trying to send as much as we possibly can to be recycled. We often have activities together with local school children to help bring awareness of the dangers of plastic pollution. We reduce our single plastic use as much as possible and all our vegetables and fruit peel leftovers are composted. We also protect the nests of turtles that hatch on our beach.

Are there any exciting changes / developments coming up in the near future?

We are partnering with Coral Eye Resort in Bangka Island so that guests can have the best service in the best areas of North Sulawesi. We are getting one more boat; we want to make sure our boats are never crowded, besides we keep working to maintain and improve all our facilities.

How do you see the SCUBA / Freediving / snorkeling industry overall? What changes would you make?

The industry is doing well and I am happy to see that there are more and more avid snorkellers and not only scuba divers. We would like to see more ocean protection worldwide to ensure the future generations get to enjoy the beautiful reef corals and marine life around the world.

Finally, what would you say to our visitors to promote the diving you have to offer?

We give you a chance to explore some of the best snorkelling and diving sites in the world while providing you with comfortable accommodation, wonderful food and the best service. Staying in Siladen Resort & Spa you can do up to four dives or snorkelling sessions every day with very experienced guides, while staying in a safe and comfortable resort that aims to provide the best service to each guest.

Where can our visitors find out more about your business?

Visit our website, find us on Instagram and Facebook (Siladen Resort & Spa) and contact us:

WA +62 811 44300641

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Into the Blue – Part Two



By now, you will have hopefully read the first blog from my recent trip to the Red Sea with The Scuba Place on M/Y Big Blue. If you haven’t, you can find the link to the blog here.

I’ve been diving since 2011, although I didn’t get really serious about diving until 2013. In the November of that year I joined Scuba School on a trip to Sharm El Sheikh to complete my Advanced Open Water course. That was the first time I heard about the famous SS Thistlegorm and its cult status in the wreck diving world. Unfortunately, as I, along with a lot of the group were novice divers, and so we were unable to dive it on that 2013 trip, along with a lot of the other famous wrecks from the North. Little did I know, I wouldn’t return to the Northern Red Sea until this trip in September 2022 with The Scuba Place. The wrecks remained mysterious all those years but I was soon getting the full experience. After the first two and a half days exploring the amazing reefs, it was time to break my Thistlegorm virginity and get the true “lust for rust” experience of the Northern itinerary.

A school of batfish greeted us on our safety stop after an amazing introduction to the SS Thistlegorm.

As we moored up at the SS Thistlegorm for the afternoon dive, I got a strange sense of anticipation run through my body. More so than at any other specific dive site. Strange really, as I don’t normally get excited about wreck diving, but here was a site that I’d heard so much about but was still so mysterious. I’d always thought it was a difficult dive and had a slight fear of it, as I wasn’t allowed to do it all those years back. Then, after watching a 20 minute film explaining the story of the wreck and listening to the stories of survivors,. I knew it was a site that demanded respect. As Mo went through the dive briefing, I quickly realised it seemed a lot more simple than I had in mind. I then became more excited than fearful as me and my dive buddy went through our plan. 

A diver explores one of the decks inside the SS Thistlegorm holding some of the vehicles onboard.

There was an eerie feeling as we submerged below the gentle swell. The visibility was a lot more milky compared to the clear blue I was used to in the Red Sea. However, the wreck soon came into view as we dropped down the shot line. The first thing that struck me and in my opinion just made the wreck extra special, was the life on it.

Instantly, crocodile fish and scorpion fish were spotted resting on the wreck, as we made our way to the anti-aircraft gun on the stern. I made a quick visit to take some photos before we turned back and penetrated the wreck for the first time. A surreal experience but the numerous glassfish and lionfish at the entry point kept me entertained before seeing the remnants of yesteryear. The different vehicles that still keep their place in the decks are the main highlight, but it was the boots that struck a chord with me: signs of the human lives that were present on the fateful day the bomb hit. I got a real buzz from my first time on the Thistlegorm, with a school of batfish greeting us on our safety stop finishing off the adventure. John and I ascended from a great dive with a high five, knowing I’d fulfilled a special memory.

A blue spotted stingray makes a quick turn on top of the SS Thistlegorm, on a memorable night dive.

I enjoyed three more dives on the Thistlegorm, giving me chance to explore a little more and see a little more life. Some cool nudibranch and a cuttlefish making their home inside the wreck added to the array of life I’d already seen. It was the night dive that truly hit the marine life spot. It really came to life at night and I soon lost count of the amount of scorpionfish I saw. The contrast of the dark and wreck against the blue spotted stingrays made their colours really pop as around six or seven were spotted. Eels, lionfish and crocodilefish making up the rest of the weird and wonderful sights on the wreck at night. Amazing memories from my first time exploring the Thistlegorm that will last forever.

After the two morning dives on the Thistlegorm, we headed off to the Barge wreck site for an afternoon and night dive. It’s not much of a wreck when you compare it to the others on the trip. It lies like a flat platform on the seabed with some sides rising out from the reef providing extra space for coral growth and marine life to enjoy. While it doesn’t provide a real wreck fix with penetration, it is a haven for marine life, littered with all types of hard and soft corals. Look closely and the Barge is a great spot for the weird and wonderful. The numerous nudibranch and grey moray eels provided my macro fix on the night dives, while the occasional buzz from huge hunting giant trevally provided the entertainment. A nice contrast of wrecks before moving on to Abu Nuhas.

The stern of the Giannis D remains largely intact and provides a dramatic underwater scene.

Abu Nuhas is a really unique place. Its submerged reef has been bad luck for five passing ships, with five cargo shipwrecks lining its northern slopes. While it was more than unfortunate for some, the wrecks have provided fortune for those looking for a wreck diving haven. Our day consisted of diving three of the wrecks  – The Carnatic, Giannis D and Marcus/Chrisoula K in that order.

Going into the trip, it was the Giannis D that I was most keen to dive. I’d always admired the wide angle stern shots I’d seen over the years, with it staying pretty much intact and creating a dramatic image as it lies on its side. It was a fantastic dive with some interesting and easy penetration; I also took some shots of the stern in all its glory. A huge grouper sitting inside the wreck provided the wildlife fix, as it floated with ease looking out into the blue from an opening on the wreck. I think it was the Carnatic that stole the show personally though. Her open windows out to the blue that are covered in soft coral were unique, and glassfish dancing in formation inside mesmerised into a truly memorable dive. The Marcus provided the adventure as penetration was a little more difficult to work my way through the wreck.

Bottlenose dolphins provided amazing entertainment as they came and played while we snorkelled at the surface.

The day at Abu Nuhas was the best of the trip for me and that wasn’t solely because of the wrecks….. YES!! Once again it was marine life that had me screaming with joy underwater and a buzz through my body like no other. FINALLY!!!!! After 9 years of taking photos underwater, I was able to share the water with dolphins (bottlenose in this instance) and shoot them in all their glory.

Our journey to and from the wrecks on each dive took us through the channel on the ribs, where dolphins were seen on every pass playing in the slight waves. After the second dive, the guides asked if we wanted to try to snorkel with them. It was a resounding yes and as the speedboat whipped up a wave storm, the dolphins headed to the surface to play. I dropped in with no elegance at all, as my excitement took over. I was wondering whether they would stay once we entered, but how they stayed and played was beyond anything I could imagine. Bringing seaweed to us and then, with a flick of their tails, speeding off after teasing with a slow approach. There were nine in total and they even came by to show off the baby of the group. It was definitely up there as one of my greatest moments in the water. 

One of three cuttlefish seen on an amazing night dive, on the house reef of Roots Red Sea.

We finished the liveaboard trip with three more amazing reef dives, with the highlight being a small cave full of glassfish and MANY lionfish. I entered to take photos of the glassfish before the lionfish started to sneak out of every crevice and reveal themselves from their camouflaged rest spots.

It got a little hairy but made for a truly interesting moment to finish the week on Big Blue. The fun wasn’t done though, as John eluded to the fact that I was on the same late flight as them on the Saturday and asked if I’d like to join his group for a night at Roots Red Sea. Sounds like a good plan!! Also, if we got there in time, a night dive on the house reef that’s a haven for the weird and wonderful would be on offer. What an amazing surprise end to the trip at an amazing dive resort: secluded, with a beautiful desert backdrop, sitting just metres from the sea. Thankfully, we made it for a night dive and it was as incredible as John said it would be. Reef squid, numerous cuttlefish, a bouncing stonefish jumping over sea moths AND a dwarf lionfish made this one of the best night dives ever, and a perfect end dive to a perfect trip. A final day of relaxation at Roots pool and enjoying the beautiful food finished it in style. 

For more information about diving on Big Blue:

Roots Red Sea lies in a secluded area of El Quseir, with a stunning desert backdrop and the Red Sea on their doorstop. It’s a perfect location for a relaxed dive trip.

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