Every few decades the Boiling Lake empties itself. It is a flooded fumarole about 60 metres across in the rainforest of Dominica, in the Caribbean, and steam-driven eruptions have drained it at least seven times, each time leaving a hole that then filled back in. The middle rolls in a permanent boil under its own cloud of steam, fed by gas and heat rising through a crack in the crater floor. Two Englishmen first recorded it in 1870, and five years later two more came back to measure it, got edge temperatures of 82 to 92 degrees and a depth over 59 metres, and could not get near the centre. No one has managed it since. The walk from the village of Laudat runs six to eight hours there and back, and it starts straight up into elfin woodland, dwarfed trees hung with moss and dripping even in the dry season.
The ridge of Morne Nicholls is the first payoff, with the park's volcanic peaks opening around you. Roughly halfway it meets a cold, clear river where most walkers stop for breakfast, boots off, bottles refilled — the last easy moment before the ground turns volcanic. Then the path drops into the Valley of Desolation, a barren basin that acid and heat have stripped of every plant, the ground stained yellow and orange with sulphur. A guide picks the line between hissing vents and bubbling mud, and you put your feet where theirs go. Crouched as close to a mud pot as the guide allows, you hold a hand over the crusted soil and the heat comes straight up through the ground. Beyond the valley the trail fords a warm river and scrambles up one last ridge. That ridge is where the path stops. The lake lies directly below, boiling in the middle and steaming to its edges, with nothing between the ledge and the water. On a still day the vapour lifts long enough to show the whole grey surface heaving, and that is still as close as anyone has got.