Koh Tachai, otherwise known as The Dome, is a very good dive site in the Similan islands in Thailand with amazing marine life and corals. It falls within the National Marine Park of the Similan Islands and Surin Islands and so is very well protected. The dive site is made up of a large dome shaped boulder sitting between 12 and 40 meters of depth and can be dived by divers of all levels. If going to the deeper sections of the dive site, down to 40 meters, divers will need to have completed the relevant deep diving certifications. Around the large dome, there are many other rocks and boulders on the sandy bottom.

There are many soft and hard corals in the area, including sea whips and some fans that create some great scenery for photographers. The reef has many tropical fish including glassfish, sweetlips, batfish, triggerfish, fusiliers, and many different types of snappers. Amongst the hard corals, there are normally anemone fish, moray eels, and butterfly fish.

Out towards the outer sections of the reef, divers can spot leopard sharks that often rest on the sandy bottom during the day. There are often large schools of barracuda and trevally lurking around, waiting to hunt the smaller reef fish. It is also becoming a well known dive site to also see manta rays and if it is the right season, whale sharks can be seen close by.