The Grotto is a collapsed sea cave under the northern cliffs of Saipan, in the Northern Mariana Islands, reached down 112 concrete steps to a pool of dark water lit blue from below by three tunnels running out to the Pacific, and it is snorkelled from the ledge and dived through the tunnels into the open sea, one of the most famous cavern dives in the world. Islands like Saipan hollow out from the inside, since the limestone dissolves where fresh water meets salt, and the Grotto is a chamber whose roof fell in, leaving a hole in the cliff with a pool at the bottom and the sea entering underneath; the stairway switchbacks down the cliff to a sunlit ledge at the water's edge, and from the surface the three submerged tunnels blaze blue when the sun rides high, from April to August, each mouth a window of daylight burning against black rock, with reef fish moving through the light. The surface pool is snorkelled without gear, and the shop-led dives go out through the tunnels to the reef beyond, where the swell is felt in the openings.
A second cavern, the Batcave, is cut into the southwest wall, completely underwater and dropping to about fifteen metres, lined past the reach of daylight with bubble coral and pink lace coral, and white-tip reef sharks often rest inside it, seen with a torch. The stairway is younger than the cave's fame, since the site was closed after typhoon damage, rebuilt and reopened, and the climb back up the 112 steps with dive gear is the price of the visit. The northern end of the island around it is the cliffs of the last stand of 1944, Banzai Cliff and Suicide Cliff, with their memorials, and the Bird Island lookout above the reef. The Grotto is a hole in the island with the sea underneath, and the light comes in through three doors.