Santa Ana, which Salvadorans call Ilamatepec, is the highest point in El Salvador at 2,381 metres, and the lake in the crater on top is closer to battery acid than to water. From the rim the ground drops away and a few hundred metres below sits milky jade, streaked white and yellow where sulphur has crusted along the shore. Steam comes off the surface in threads and out of vents in the crater wall, the wind carries a sour bite up to you, and the water down there runs past 60°C. That crater is barely a century old. It opened around the eruption of 1904, and it sits inside an older one, which sits inside an older one again: this mountain has collapsed and rebuilt itself over and over, every new vent breaking out within the scar of the last. You can read the whole sequence from where you stand. Terraces and broken ledges mark the lips of earlier craters, and walking a little way round the rim lines the layers up, red, grey and sulphur-yellow.
The walk up takes about two hours from the cloud forest at Cerro Verde. It starts in dripping forest, thins into scrub and ends on open volcanic gravel where nothing grows at all, steady rather than brutal, with more of western El Salvador unrolling behind you the higher you get. Turn round at the top and the country is laid out: Lake Coatepeque filling its own caldera, a strip of Pacific on the clearest mornings, and Izalco standing out of green forest as an almost perfect grey-black cone. Izalco is the strangest of them. It grew out of flat farmland from 1770 and then burned almost without pause for nearly two centuries, steadily enough that sailors steered by its glow and called it the Lighthouse of the Pacific. In the 1950s a hotel went up on Cerro Verde with its windows framed on the cone. The eruptions stopped in 1958, just about as the doors opened, and apart from one flank eruption in 1966 it has been dark ever since.