This dive site description is for Boulder City, in the Similan Islands, Thailand. Boulder City is located in the south of the Similan chain and is regarded as a deep and challenging dive for PADI Open Water and PADI Advanced divers. Professional divers will enjoy the topography, visibility, and the currents.

• 10m-40m
• Fantastic visibility
• Currents can be strong
• Looks like a huge loaf of bread

Scuba diving Boulder City is a fantastic experience for lots of things, except coral.

However, don't start thinking that there's nowhere for fish and invertebrates to hide or feed, because there's lots of algae and some quite interesting currents,

which bring food directly from the north or the south, depending on the time of day. The marine life here is healthy, colourful and diverse.

Firstly, one needs to understand the topography a little. Boulder City gets its name from the collection of massive granite boulders which are sitting on the sandy sea bed at a depth of approximately forty metres. Some of these rocks are as big as a house, and their dispersal is so neat and orderly in places that the whole dive site looks like a massive loaf of sliced bread. The shallowest point of Boulder City is 10-12m, depending in tides, and some of the boulder tops are as deep as eighteen metres. Being completely submerged means that buoy lines are attached at the north and south ends of the dive site. The buoy line at the north end can take you down to 20m, and the southern line 30m. Slightly further away from these boulders the seabed drops away to fine sand. Currents can be strong at times, and with so much open water between the surface and the rocks, divers need to be fit, alert and well-prepared. Any respectable dive tour operator will plan the dive sensibly, but the unfortunate truth is that the visibility is greatest and the best marine life is there to be seen when currents are strongest. Its depth is often a reason for it being the first dive of the day for Similan Island liveaboard diving boats. Being located at the southern end of the Similan Islands, it is often among the dives on liveaboard trips' first or last days out at the Similans.

Its location is about a kilometre to the east of Similan Island #3, which is locally known as Koh Payan, and just a few hundred metres south from Sharkfin Reef, which is among the best Similan scuba diving sites. Diving tours in Thailand are usually well-organized and able to adjust their plans to suit conditions. Therefore, if discovering too many other dive boats or unfavourable conditions upon arrival, there is always plenty more dive sites to visit nearby.

Marine life at Boulder City is amazingly diverse, healthy and colourful. The lack of coral is not a problem because there's plenty of algae to feed on and food passing in the currents. Gorgonian Sea Fans have made their homes between some boulders, to make the most of so much passing plankton. Divers need to have eyes everywhere, because although there's more to take in than you can see in one dive around the boulders, it's always a good idea to keep an eye out into the clear blue water for something large. The gentle giants which visit or cruise by Boulder City are the likes of Whale Sharks (Rhincodon typus), Manta Rays (Manta alfredi), and Eagle Rays (Aetobatus narinari). However, please don't expect to see these, or be disappointed not to. There is so much more here, including mid-size fish patrolling in schools and alone. Oriental Sweetlips (Plectorhinchus vittatus) are colourful and not at all bothered by divers. Titan Triggerfish (Balistoides viridescens) can be aggressive over their nests in the sand. Purple and red gobies are interesting smaller fish here, and one species, the shy Andaman Jawfish, is actually endemic to the Similan Islands. Divers will see scores of Unicornfish (Naso), Butterflyfish (Chaetodontidae), and Angelfish (Pomacanthidae) grazing on the algae on each boulder. Humphead Parrotfish (Bolbometopon muricatum) and Napoleon Wrasse (Cheilinus undulatus) are larger residents commonly reported here at Boulder City. Here we have mentioned but a few of the many fish species to be seen at Boulder City and other Similan Islands diving sites.

Lionfish (Pterois) above - photo by Jonas Fagerberg

Where is Boulder City?

Please feel welcome to leave comments on our review of Boulder City, and this applies to all of our Similan Islands diving reviews. We try our best to as accurate as possible, but cannot be at every dive site in Thailand all the time. In an ideal world, you divers will report your experiences and sightings of these dive sites in our comments box below this text or join and exchange information in our forum.