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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


Scotland Underwater



The first in a series of blogs about Scotland Underwater from Ross Mclaren…

Here in Scotland our driech and dreary weather is world famous. But actually, the copious amount of rain that we often moan about, is responsible for a cacophony of colours across our beautiful country.

The one place that might not always be as renowned for being vibrant and colourful is our seas and lochs.

As always there’s exceptions. Our beaches on the north west coast are covered in golden white sand and with turquoise water that might be mistaken for the Maldives… albeit a wee bit nippier… and we’ve even got a few wee lochs (called Lochans) with some pretty green shades to them, but for a good percentage of our coast and lochs, it’s a steely grey mass that greet us.

So, you’d be forgiven for thinking that Scotland’s underwater world mirrors the water it lies beneath. Now, I’m not going to pretend you’re going to be met with a rainbow of colours found somewhere like the Great Barrier Reef, but actually the vibrancy found under the waves definitely took me by surprise.

Disclaimer! I’m no expert in marine biology or underwater photography! I’m pretty much a guy with “all the gear and no idea!” I started out with a wee GoPro and built my camera “rig” up to something that’s now resembling an octopus. But, I’ll be completely honest, I have no real clue what I’m doing in terms of settings, etc. It gets put on “Auto”, I turn the lights on, try not to disturb the marine life and press the button hoping for the best. Quite simply, I’ve fallen in love with our underwater world and do my best to try do it some justice through my photos.

One of the most beautiful marine species I find photographing, and to be honest probably one of the easiest, is the anemones. We have such an abundance of these from deadmens fingers, to firework anemones, and the colours that can be found are just breathtaking. The patterns and shapes they make as they glint in the light of the torches and with the movement of the water is magical.

They might not be the most exciting sea creatures but the humble crab is also a fantastic specimen to capture, and again we have a wide variety. I’m not quite sure what it is but you can almost see/feel the attitude oozing out of them when you catch them in the beam of the lights.

I say this to almost anyone who’ll listen, but I always said I would absolutely love to get sweeping wide angle photo of a wreck. Those are by far my favourite photos to look at. Seeing these hulking feats of human engineering being reclaimed by nature and appreciating the scale of them in one scene is awe-inspiring. Sadly in Scotland with our visibility (well certainly in the areas I frequently dive) it’s not really possible and when it is, it really doesn’t do it justice. However on the flip side Macro photography here is definitely rewarding!

Last summer I had one “photographic goal”… get a nudi! I was desperate to capture a wee sea slug, but no matter how hard I looked I could only find one all year and when I did my GoPro just didn’t do it justice. This year though, well it seems to be a completely different story! Every dive we seem to come across at least one… it also helps when you’ve an eagle eyed dive buddy! With the new camera and macro lens the quality in photos has improved as well. It’s not just the number we’ve seen but the variety we’ve spotted as well! There are so many different kinds, different colours and shapes. It can be a wee bit frustrating trying to hold myself still in the water and getting the camera to focus in on this tiny wee creature, but it’s so worth it!

The dogfish/catshark isn’t particularly uncommon in the UK and it’s no different up here in Scotland, if you know where and when to look. They are absolutely stunning to photograph and, although not overly colourful, the texture of their “skin” and their eyes is absolutely incredible.

Now cucumbers are most definitely not my favourite vegetable… but sea cucumbers… those I do love! I genuinely can’t get over how cool they look. They remind me of wee trees and I’m totally mesmerised watching them bring the food to their mouths with their tentacles.

Jellyfish! The scourge of beach goers everywhere! The dread of someone shouting “JELLYFISH” and hoping beyond hope you aren’t caught in a tentacle brings back childhood memories. So until I started diving the “evil” jellyfish was much feared. However, since I started exploring the underwater world and seeing them in all their glory, I have come to appreciate jellyfish for they unbelievable beauty and grace. I love watching them float past (from a distance!) and seeing the shapes they take in the water. They are so full of grace!

Even the most dived sites can throw up a wee surprise every now and again. We’d headed to one our usual haunts with the main goal of logging a couple of deeper dives just to build up to Scapa later in the year. We descended down to around 38m where we planned to swim along for a wee bit before ascending again. There were a few rocks, but generally not much life but I took the camera anyway, you know, just in case.

Now these guys aren’t completely uncommon here on the west coast, but they’re mainly found at night and until now I’d never spied one, let alone photographed one! Bobtail Squid/Little Cuttlefish! I’m not going to lie, I was so excited! I actually thought I was slightly narked as it appeared out of the sand. This wee fella was so cool! The colours were absolutely breathtaking and getting the opportunity to photograph them was just amazing.

Scotland isn’t the diving capital of the world; we’re not going to suddenly become a top dive destination on many diver’s bucket lists. BUT we do have some incredible marine life, with such unbelievable colours! Although it’s not the easiest diving you’ll ever do, when you do get that moment it makes it feel all the more special.

For more from Ross, follow him on Instagram @underwater.ross and on Twitter @outdoorsross.

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A Red Sea Scuba Scene (Part 2 of 2)



The first two days of diving were amazing – I think you’ll agree after reading Part 1 of the blog HERE. We left the Brothers Islands setting sail for Daedalus – the southernmost point of our itinerary around 275km southeast of Hurghada. Conditions were perfect for our crossing and continued throughout our day at Daedalus for three dives. I was so excited for this site as it was the highlight on my previous trip and I’d also had word it was the hot spot for oceanic whitetips the last couple of months.

A feeding hawksbill turtle pays no attention to the divers on Daedalus.

We moored up by the lighthouse at the southern point of the island and was thankful to see there weren’t as many boats as at the Brothers. Our first dive was a rib dive to the North Point to drift out in the blue at around 25 metres+ in the hope of seeing scalloped hammerheads. I wasn’t expecting the same action as my previous trip with schools of around 20 hammerheads due to difference in the time of year and sure enough the action didn’t hit as big. We spotted a couple of lone hammerheads between the group deeper than 40 metres. After spending half the dive in the blue we came back to the stunning East wall with its amazing soft coral and small fish life. Towards the end of the dive we had an incredible encounter with a feeding hawksbill turtle that was completely comfortable with our presence as it fed on the soft coral. It’s always a pleasure seeing turtles.

Oceanic whitetip shark swimming under the sun at Daedalus.

Although we were on the rib once the dive was finished, the action wasn’t over. As we neared Scuba Scene we saw some commotion with other ribs in front stopping and looking in the water. Initially the rib skipper said it was a whale shark but as we neared we saw the unmistakeable dorsal fin of an oceanic whitetip shark break the surface. In fact, there were two of them and they were really excited. I lent over the side with my camera and got my best photos of them as one came to investigate bumping into the camera. This is what I love; this is what gets me excited and sure enough for the next two dives I decided to stay under our boat at around 5 metres for most of the dives. There were three in total around Daedalus and I had some incredible close-up encounters with them. This is what I was here for and I was so happy after our day at Daedalus with the oceanics.

Although the conditions at Daedalus were like glass, the weather forecast wasn’t looking great for the next two days and the decision was made to journey back north to Elphinstone instead of staying for another day at Daedalus. I was a little disappointed as it would mean missing out on some more great shark action. However, I missed out on Elphinstone on my last trip due to bad weather and was happy to get the chance to dive there finally.

Lionfish swimming amongst the stunning reef of South Plateau, Elphinstone.

Sure enough the winds picked up during the night and it was a lot more choppy when moored up at Elphinstone. With Scuba Scene’s size, it was very capable of dealing with rougher seas and we planned for a full day there. We had two morning dives before deciding to head inland as conditions worsened. My dive buddy and I stuck with the South Plateau for the two dives and both were stunning. The life on the plateau was amazing as lionfish were in abundance and while photographing them I got surprised by my very first torpedo ray. It was only a juvenile and what a cutie it was as it swam over my dome and turned just before it hit me and swam away. Two friendly hawksbills were again a highlight as they didn’t care for the divers exploring the plateau. While ANOTHER oceanic whitetip really made our trip to Elphinstone in bad weather worthwhile. FIVE different oceanics on the trip; I was happy to just get one but buzzing with the action at three different sites.

Colourful pyjama nudibranch on a night dive at Abu Dabab 3.

It wasn’t all bad leaving Elphinstone early as we managed to get an extra dive in with a night dive at Abu Dabab 3 after an afternoon dive there also. The afternoon dive was a highlight of the trip for me as I got to experience something different with a “cave” dive of sorts. My dive buddy sat the dive out but guide Adma Rashed was eager to get in as he loved exploring the caves. I was soon following him exploring a shallow cave system through the reef. As it happens, this was his first time exploring the whole way through the system and he was so happy after the dive. I’m no cave diver and have no interest in deep cave exploration but this was really fun and different to everything else on the trip. I’d certainly like to do more of this relaxed type of cave diving.

One of the many moray eels at Small Giftun Island.

The rest of the trip for the Thursday and half a day on the Friday was Red Sea reef heaven again. A night dive at Mangrove Bay provided a couple of cuttlefish (I love cuttlefish) and also my first time seeing a Spanish Dancer underwater. Although we tried the seagrass at Marsa Shona and saw a green sea turtle from the surface, we couldn’t find any underwater and soon left to explore the reef – an amazing reef full of blue spotted ribbontail rays to enjoy. We finished with two dives at the Police Station dive site around Small Giftun Island. The gorgonian fan corals were a beautiful sight but the highlight of diving here were the huge moray eels and, in particular, one huge free swimming moray that swam next to me for a brief period right at the end of my last dive.

WHAT A WEEK OF DIVING!!!! Thank you Scuba Scene Liveaboard and Oyster Diving.

Exploring a shallow cave system at Abu Dabab 3 was a real highlight.

Sean Chinn travelled as a guest of Scuba Scene Liveaboard and Oyster Diving. Scuba Scene is available to book exclusively through Oyster Diving. Please contact or call 0808 253 3370 to find out more or reserve your space!

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A luxurious dive resort in the heart of Lembeh Strait. Enjoy refined services while exploring the rich waters of Indonesia.

The resort is nestled around an ocean front deck and swimming-pool (with pool-bar) which is the perfect place to enjoy a sundowner cocktail at the end of a busy day of critter-diving.

All accommodation is full board and includes three sumptuous meals a day. Breakfast and lunch are buffet meals and in the evening dining is a la carte.

Book and stay before the end of June and benefit from no single supplements in all room types!

Booking deadline: Subject to availability – book and stay before end of June 2022

Call Diverse Travel on 01473 852002 or email

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