Mount Yasur has exploded every few minutes for about eight hundred years, and you can walk to the lip of it and watch that happen. The cone is 361 metres high, on the island of Tanna in Vanuatu, in the South Pacific. Its vent stays almost permanently open, so gas escapes in small constant blasts instead of building toward one huge eruption, and that is the whole reason you can stand this close. The way in crosses a plain of bare grey ash, blasted clear of almost all life and steaming in patches, with the cone rising out of it. It is a stark stretch of ground that feels a world away from the tropics on every side of it. By daylight you look straight down from the lip at two or three vents smoking far below, and catch the dull red churn of molten rock between the explosions.
The half-hour of blue dusk is what everyone climbs for. You reach the rim while the sky still holds light, and then the colour drains out of the cone and the ash until the crater's orange pulse is the only thing left. Once it is properly dark the vents let go every few minutes, throwing ropes of glowing lava high into the black before they clatter back down the slope. The boom reaches you a beat after the flash, and you feel it in your chest. That glow carries a long way out to sea: it was what turned Captain Cook towards Tanna in 1774, and sailors came to call the mountain the Lighthouse of the Pacific. The ash route up is also a sacred one. The villagers of Sulphur Bay, who follow the John Frum movement, believe a spirit lives inside the crater with the souls of the dead, and they walk up this way to it. You come back down the same ash in the dark, and the booms keep arriving behind you, on the beat the mountain has held for eight centuries.