The Grotto is a collapsed limestone cave that the ocean now fills. It lies beneath the cliffs of Marpi, at Saipan's northern end, where the roof of a chamber fell in and left a jagged sinkhole holding a pool of deep blue seawater. The pool is joined to the open sea by three flooded tunnels, spread between the surface and 18 metres down, which run straight out to the Pacific. Those tunnels are what makes the water glow. The light arrives from below rather than from the sky. Daylight crossing the sea outside loses its red wavelengths first, so only the blue end of the spectrum survives the passage through the tunnel mouths and floods the chamber. A hundred and twelve concrete steps switchback down the cliff to the waterline, and once you are under, visibility routinely runs past 30 metres, the clearest water Saipan has.
Getting in is a matter of timing. At the foot of the steps you edge onto the flat entry boulder that divers call Penguin Rock, wait out the surge that washes over it in sets, and giant-stride into five metres of water. The way out is to hold the buoy line and let a wave lift you back onto the ledge. Below, one or two white-tip reef sharks hold station near the tunnel mouths, and green turtles and Napoleon wrasse cross the light. A second cavern, cut into the cliff wall at 12 to 15 metres and known as the Batcave, glitters with pink lace coral. The current that keeps the water so clear is also what makes the site deadly: surface outside the tunnels and the waves pin you against sharp rock with no way back in, so almost everyone dives here with a local shop. The island itself is still recovering. Super Typhoon Sinlaku tore across Saipan in April 2026, flights only resumed in June, and Marpi's dive sites are reopening one at a time.