Lake Lalolalo is a circle of black water inside sheer walls fifty metres high, on Wallis Island in the South Pacific. The track in leaves the road and closes over with trees — a short, humid walk on red earth, loud with insects. It gives no hint of what is coming until the forest floor simply ends. A plain concrete slab and a low fence are the only built things here, and the wall goes straight down into water nobody swims in. Nothing eroded this. Magma rising through the island met groundwater, the water flashed to steam, and the blast blew a shaft out of the ground in one go. No volcano was ever involved beyond that.
Around midday the sun climbs high enough to clear the wall and light the whole basin instead of shadowing half of it. That is the hour to be on the lip. White-tailed tropicbirds work the air inside the crater, which puts them under your feet rather than over your head. From the rim you look down on white backs and trailing tail streamers turning against the black water. The lake goes down 88 metres, and it is two different waters. The top is fresh rain; near the floor it is nearly as salty as the sea, because the crater cuts below the lens of fresh water inside the island. The oxygen runs out about ten metres down, so everything alive is crowded into the thin lit skin on top. The forest you walked through is held, with the lake, under vao tapu, the customary law that has kept it standing. Those walls have kept something else. Wallis was a major American staging base in the Pacific war, and when it ended the surplus was reportedly pushed over this rim rather than shipped home. Local accounts talk of vehicles and heavy machinery; what divers have actually brought up is wartime ammunition. Fifty metres of cliff make the water hard to reach, and the bottom has never been properly searched.