You meet the cave before you enter it. A doline — a sinkhole where the limestone roof has collapsed — opens a panoramic view down over the entrance and the first two of its skylights, so the introduction is aerial rather than subterranean. From there the claraboias keep coming, spaced out along roughly three and a half kilometres of route. Each one lights a patch of floor, and each patch grows something different depending on how much sun it gets and for how long. The effect people reach for is a Japanese garden: not lush, not jungle, but arranged — small, particular, oddly composed, plants placed by physics into groupings that look deliberate. Between the skylights the dark closes back in, and the contrast is the point. You walk out of a lit room into a black one and then into another lit room.

Janelão runs 4,740 metres in horizontal extent with 176 metres of vertical relief, which makes it the largest cave in the Peruaçu valley. Those figures do very little until you are standing in one of the galleries, where the ceiling and the walls can both be more than a hundred metres away. A cathedral would fit inside with room left over, and unlike a cathedral nothing here is scaled to a human congregation. Hanging from one of those ceilings is a stalactite measured at 28 metres — a single mineral icicle nearly as long as a ten-storey building is tall, deposited one drip at a time over a span nobody watching it could comprehend. Locally it goes by the Perna da Bailarina, the ballerina's leg, which is the sort of name that only occurs to people who have looked at something long enough to stop being frightened by it.

The Peruaçu River does not pass by the cave; it passes through it. This is the stretch where the river crosses a limestone massif that once blocked its path, and the water you hear inside is the same water that has been dissolving the rock into these rooms for as long as there has been a river here. It runs the full internal length of the cave, and it is as much of an attraction as the formations. Cave rivers behave differently from surface ones. Sound carries strangely, the temperature holds steady, and in the lit sections under the skylights the water goes an improbable green before disappearing back under the ceiling. The cave is not a container the river happens to occupy. The river is the tool that made it.

Janelão sits in the municipality of Januária, inside Cavernas do Peruaçu National Park, and carries the catalogue code MG-199. Visits are guided and scheduled, which is not bureaucracy for its own sake: speleothems that took millions of years to form snap off in an instant, and the boot-worn margins of a trail through a cave never recover the way a forest path does. This is also the country of the Peruaçu rock paintings, some of them more than ten thousand years old — Janelão is part of a system where people have been showing up for a very long time. Whatever the modern visitor is doing here, they are doing it late. The three-and-a-half-kilometre route takes the better part of a day, and there is no version of it that rewards hurrying.