Lying just to the north of Koh Bida Nok is the little sister, Koh Bida Nai. Another favourite of divers visiting the area, Ko Bida Nai is a limestone cliff islet with similar topography to it's larger neighbour and hosting equally diverse marine life.
Divers often descend into the shallow bay to the east of the island onto the sloping reef and coral encrusted boulder formations which lead past many coral bommies covered with anemones to the first of several swim-throughs.
Created by a huge slice of fallen cliff-face many thousands of years ago, this large gap in the rock is not immediately obvious unless you knew it was there.
Often filled with thousands of tiny glass fish that disperse as you begin your swim-through, keep a close eye on the rock walls for colourful nudibranchs and take care with your buoyancy as scorpion fish may also be found on the bottom.
Exiting the swim though and heading towards the left brings divers to Fantasy Reef, covered in barrel sponges, whip corals, gorgonian sea fans and a huge variety of marine life. Schools of jackfish and trevally are often found hunting on bait balls of younger fish above the reef.
After Fantasy Reef, further back up on the shallower parts of the reef to the north are found several very large rocks with the second swim-through and a long sloping reef which continues to the north west of Bida Nai where it becomes a wall around the top of the island, connecting back with the original entry point.