Diamond Cave / Phra Nang Nai Cave

Slip off Railay’s sand and into the cool dark of Diamond Cave, a short walk inland from Railay East. A lit wooden boardwalk winds you past curtains of stalactites and thick columns that gleam like—yes—diamonds when your eyes adjust. The air drops a few degrees, you catch a whiff of damp limestone, and if you look up, you’ll spot a resident bat colony clinging to the ceiling. It’s an easy, self-guided loop with railings most of the way, but watch your step—some planks are uneven and the floor can be slick after rain. Plan 20–30 minutes inside. Entry is 200 THB (~$5.50) per adult, paid at the small ticket booth. Go early if you want it quiet; by late morning small groups start filtering in from the beach. Practical bits: open daily 8am–5:30pm, and sneakers or grippy sandals beat flip-flops here. Combine it with Princess Cave on Phra Nang and a sunset on Railay West for a tidy half-day circuit when the sun is at its fiercest.