Mount Rinjani is a 3,726-metre volcano on the Indonesian island of Lombok, and three days of climbing get you not a summit but the edge of a hole. Something far larger stood here until 1257, when it emptied its magma chamber in one of the largest eruptions in recorded history, caved in, and cooled the whole planet. For seven centuries nobody could say which mountain had done it. The blast was pinned to this one only in 2013. What it left is a bowl six by eight and a half kilometres wide, with Segara Anak on its floor, a turquoise lake over 200 metres deep. A young cone called Barujari rises straight out of that water, and it is not finished. It last broke the surface in 2016, clouding the lake grey with ash.
From Sembalun in the east the way up crosses open golden savanna that tips up towards the rim; most people come down the other way, through Senaru's rainforest. Camp is pitched on the crater edge, the ground dropping hundreds of metres to the water below you. As the day ends the walls turn copper and the smoke off Barujari catches the last of the light. With no town for miles, the dark that follows is deep enough for starlight to throw faint shadows. The summit push leaves around two in the morning, up loose ash that gives back half of every step. You top out as the sun comes up on an unbroken floor of cloud, the volcanoes of Bali on the far horizon. You do not have to go up at all. A steep path drops the other way, off the rim to the lake shore, where warm water pools between the rocks at Aik Kalak. The Sasak call the mountain's guardian Dewi Anjani, a spirit-queen who rules summit and lake together. Around a full moon, pilgrims make that same descent and cast small gold fish into the water. Barujari stands in the same lake, still working, putting the mountain back together inside the hole it made.