Lake Toba fills the crater of a supervolcano in the highlands of North Sumatra: a hundred kilometres of water, the largest volcanic lake there is, with walls standing five hundred metres above it. In the middle there is an island, Samosir, as large as Singapore. It is not a leftover piece of shoreline. Samosir is the caldera's own floor, domed back upward after the eruption by magma that never stopped pushing, and it is still rising. The eruption was about 74,000 years ago. One idea has its ash darkening skies worldwide and cutting early humans down to a few thousand, though later work found no clear sign of that, and the argument continues. The roll-on ferry from Parapat takes forty minutes, the caldera wall sliding past the rail beside you.
Samosir is the Batak homeland. Their old houses carry saddle roofs that sweep up at both ends like a ship's prow, the timber joined without cement and set on bare stone. The gables are covered in carving painted red, black and white, for the earthly, spiritual and divine worlds. At Tomok and Ambarita the stone sarcophagi of royal families stand between the houses, along with a ring of carved stone chairs where kings held council and judged the accused. From a viewpoint on the rim the lake fills the horizon, Samosir floats low in its centre, and the shoreline villages are specks hundreds of metres below. In the first hour of light there is no wind, and your boat finds the whole rim repeated upside down on the surface, mist sliding off the slopes. Late in the day the sun drops behind the western wall and the water goes from blue to copper around you. The water is cool and clear at nine hundred metres, and you swim in it with five hundred metres underneath you and the floor of the crater, still on its way up, behind you on the shore.