Swim-through (passage in rock)

A swim-through is a natural opening or tunnel within a reef or rock formation. Divers pass through these gaps to reach new areas of a dive site. You often find them in limestone formations like those at Anilao or the coral overhangs in Koh Tao. These passages can range from 0.5 m wide to massive caverns that hold several divers at once.

Watch your fins. Many divers hit the ceiling or floor because they forget how much space their gear actually takes up. I once saw a diver get stuck in a narrow passage near Cebu in 2018 because his tank snagged on a coral edge. Always maintain neutral buoyancy before you enter any tight gap. This prevents you from accidentally kicking the delicate reef structure while you move through the tunnel.