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Boulder City

Napoleon-or-Humphead-Wrasse-Cheilinus-undulatus-at-Boulder-City-Koh-Similan-Thailand-Henry-and-TersiaNapoleon or Humphead Wrasse (Cheilinus undulatus)
Photo by Henry and Tersia
Boulder City obviously gets its name from the hundreds of huge granite boulders and rocks that make the dive site what it is. It is the southernmost diving site in the Similan Islands and often visited for one of a liveaboard trip's final dives. Boulder city is quite deep, and not for the faint-hearted, but an excellent dive site and can give divers some very nice surprises.

  • Extremely good underwater visibility
  • Looks like a massive loaf of sliced bread
  • 40 metres deep
  • Good chance of seeing something big

Butterfly-Fish-Chaetodontidae-at-Boulder-City-Koh-Similan-Thailand-Lyle-TurnerButterfly Fish (Chaetodontidae) Thanks to Lyle Turner at the Global Reef Project Divers will enter the dive site at either the north end (right next to the southeastern tip of Sharkfin Reef) where the mooring is attached to the reef at 20 meters, or at the deeper south end. This decision will depend on the currents, which can be strong at Boulder City. Some divers will use the line to get all the way down to the bottom then use the rocks and boulders for shelter as well as exploring for all kinds of marine life. There isn't a great deal of coral at Boulder City, but this doesn't reduce the quantity or quality of fish and invertebrates there. The massive granite boulders are spread out in an unnaturally neat way, making the dive site appear to be a huge loaf of sliced bread, with large rocks on the sandy seabed being the crumbs. Visibility is normally excellent, especially during the stronger currents, so if something large swims by, there's a better chance of seeing it. Manta Rays (Manta Alfredi) and Whale Sharks (Rhincodon typus) pay visits from time to time, especially late in the diving season, but are by no means guaranteed. Reef sharks and smaller rays can usually be found out on the sand which surrounds the rocky dive site.

Boulder City diving mapThe lack of coral is not a problem, because in its place is a vast covering of algae, attracting grazing fish and invertebrates and, consequently, those who prey on them. Some of the larger bony fish to be found both here and at Sharkfin Reef are Humphead/Napoleon Wrasse (Cheilinus undulatus) and Humphead Parrotfish (Bolbometopon muricatum), both of which are slow, steady and solitary swimmers. Larger fish in schools include Barracuda (Sphyraena) and Snapper (Lutjanus). Butterflyfish (Chaetodontidae), Angelfish (Pomacanthidae), and Unicorn Fish (Naso) are among the more colourful and medium-sized reef fish grazing on the algae here. Such good visibility is not just an advantage for seeing fish far away. Those who enjoy underwater photography can benefit from getting the most amount of light, and therefore colour when snapping away at the colourful fish and nudibranchs which can be found all over the granite rocks and boulders.

Sheltering under the deeper areas, Oriental Sweetlips (Plectorhinchus vittatus) are colourful and don't appear to be afraid of being approached. Giant Moray Eels (Gymnothorax javanicus) are the most popular of their type, but other more-colourful species are there, too. For those who know what to look for and identify, it's possible to see a species which is endemic to the Similan Islands. The Andaman Jawfish (Ospitognathus albicaudatus) is neither exciting nor attractive, but it's new to science and only lives here.

A dive at Boulder City might not be as long as at other Similan diving sites, due to depth and possible currents, which is a shame, because it is a huge dive site, covering a very large area of seabed. However, the amount of colourful fish and invertebrates which graze on the algae should make up for it. Only liveaboard boats and the occasional speedboat day trip visit Boulder City at the Similan Islands, and we offer the best prices for all available packages, including those with non-diving partners or children. To find the most appropriate package that meets you budget click below.

Have you ever dived at Boulder City?
If you would like to let the diving community know about your diving experience at Boulder City then add a comment below.


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