The Grotto is a collapsed sea cave under the northern cliffs of Saipan, in the Northern Mariana Islands, and the blue in it comes from underneath. Three drowned tunnels connect the pool to the open Pacific, so the light that fills the chamber has crossed the sea first and arrives with its reds stripped out. The way in is a stairway of 112 concrete steps switchbacking down the cliff, steep, humid and slick with spray. Partway down the chamber opens up and you realise how big it is. At the bottom a sunlit ledge runs along the water, and the pool can be snorkelled with nothing but a mask: you float on the surface and look down at the three tunnel mouths burning against black rock. The blue has a clock on it. It needs the sun high enough to drive light through those mouths, so it is a midday thing, strongest from April to August; once the cliff shadow arrives, the chamber greys out.
Divers go out through the tunnels to the open sea, and that is guided work, not a swim you invent. Outside, the current that keeps this water so clear runs hard, and the only way back in is one narrow throat. Anyone who surfaces beyond the mouths is held against rock too sharp to climb; it has killed divers and snorkellers here. A second chamber, the Batcave, is cut into the southwest wall, entirely underwater, dropping to about fifteen metres and lined past the reach of daylight with bubble and pink lace coral. White-tip reef sharks lie up inside it, and the cave fish throw your torch beam back as pairs of eyes. The stairway is younger than the cave's fame. A typhoon wrecked the railings in 2018, and the site stayed shut for more than a year before new stairs reopened it. The chamber itself was dissolved from inside the island, where rainwater sinking through limestone floats on seawater and the boundary between them eats rock faster than either does alone. It had no entrance at all. The steps exist because the ceiling gave way.