Koh Ha Yai

Koh Ha Yai (meaning "big") is the largest of the five Koh Ha islands, situated between Phi Phi and Hin Daeng, close to Koh Lanta in Thailand. The island features a steep cliff face that drops perpendicularly from the surface down to around 30 metres, where the terrain levels out to a sandy bottom scattered with soft corals, sea fans, and hard corals. A rock wall runs along the south side of the island, and the reef surrounding the island is composed of hard coral, with coral heads and fields of sea whips found on the reef to the west down to 20–25 metres.

The defining features of Koh Ha Yai are two large underwater caverns on the south-east side. The entrance to the first cave sits at around 12 metres and narrows as you go deeper — cave penetration is not recommended unless you hold the appropriate certification. The second cavern, known as the "Cathedral," lies to the east; its entrance is split in two, spacious and well lit by light reflected from the surface. Inside, divers can surface and look up at large stalactites. At the back of the east cavern a small hole opens into a tunnel roughly 25 metres long that ends in a chamber, and this section should only be attempted by divers with proper training and equipment. A good dive plan starts on the reef to the west of the caverns before moving east to explore them.

The site suits beginner divers, with a maximum depth of 30 metres, light current, and water temperatures between 27–30°C. Visibility is generally great. The best diving season runs from November to April.