Caves are supposed to stay where they form. This one did not. Caverna dos Ecos began the way nearly all big caves begin, in soluble carbonate rock - limestone and marble, hollowed out by groundwater over an unhurried stretch of geological time. As the void widened, the roof stopped holding. Blocks came down, the ceiling worked its way upward into the mica schist and quartzite lying above the carbonate, and the cave effectively migrated into rock that water could never have dissolved in the first place. Around 70 per cent of its volume now sits in schist and quartzite. That is peculiar enough to have earned a scientific paper of its own: no other Brazilian cave of comparable size is known to have developed in these lithologies.

The lake at the bottom is fed by rain working its way down through the mountain, and it is genuinely large - roughly 300 metres long, 50 across, and between 10 and 15 metres deep. Brazilian sources routinely call it the biggest underground lake in South America. The environmental agencies phrase the superlative more carefully, as the largest known lake cavern in mica schist and limestone rock in Latin America, which is a narrower claim and an easier one to defend. Either way, standing at the water's edge with a headlamp produces a very particular sensation. The far shore is beyond the reach of the beam. The surface is glass-still, because nothing down here disturbs it. And the cave earns its name: sound travels out across the water and comes back altered, which is presumably how it came to be the Cave of Echoes.

Geologists reserve the word karst for landscapes shaped by rock dissolving into water - the sinkholes, blind valleys and cave systems that limestone country reliably produces. Caverna dos Ecos is described as a karstic feature, and yet the great majority of it sits in rock that does not dissolve at all. The resolution lies in the sequence of events. The dissolution came first, down in the marble and limestone; the collapse came afterwards, carrying the void upward into schist and quartzite that only had to break rather than be eaten away. It is a karst landform living at the wrong address, and that is precisely why speleologists keep coming back to it.

The cave was discovered in March 1975 - remarkably late for a void 1,380 metres long lying less than a hundred kilometres from Brazil's national capital. That lateness says something useful about the Planalto Central. Brasília had been inaugurated only fifteen years earlier, and the cerrado around it was, in most practical senses, still unmapped. The entrance sits at 1,050 metres of elevation near the settlement of Girassol, in the municipality of Cocalzinho de Goiás, just past the Federal District boundary and 134 kilometres from Goiânia. In the national speleological register it carries the code GO-018.

This was never a walk-through cave or a casual afternoon. The route drops 142 vertical metres, and the schist is loose in places where the roof has failed - which is, after all, how the cave came to be where it is. General tourist visitation has in fact been closed since 2001, when Brazil's environmental agency IBAMA issued an order banning tourism access after the cave suffered visible degradation from foot traffic; entry today is restricted to authorised scientific and speleological expeditions. What those expeditions find is a kind of scenery that exists almost nowhere else. No formations, no ochre and cream flowstone, no decoration of any kind. Just enormous plates of mica throwing back a lamp beam in flashes, the dull ring of your own voice returning off the water, and a black lake that has been quietly filling since long before anyone knew it was down there.