The rock is dolomitic limestone, and geologists working for the state park will tell you it was laid down on a sea floor roughly 600 million years ago; seashells still turn up in the surrounding country, a long way from any coast. Everything since has been the slow arithmetic of water and stone. Rainwater picks up carbon dioxide, turns faintly acidic, and spends geological time dissolving carbonate along cracks. The Lapa River did the coarse work, cutting a passage that runs some 5.8 kilometres through the hill, and the drip did the fine work, hanging stalactites from the ceiling and building stalagmites to meet them. At some point in the distant past a landslide split the system in two. The first half is walkable. The second, called Terra Ronca II or Malhada, needs a guide, and rewards one with skylights, sand dunes heaped inside the rock, and a chamber known as the Salao dos Namorados, the Lovers' Hall.

Every year at the beginning of August, the entrance becomes a church. The Romaria do Bom Jesus da Lapa da Terra Ronca draws pilgrims from across the sertao for about a week, and the ceremony happens where the daylight ends and the cave begins: open-air mass under a ninety-six-metre ceiling, baptisms, weddings, and a procession of the faithful walking into the dark carrying candles. It is a devotion with deep roots in the Brazilian interior, borrowed from the great cave shrine of Bom Jesus da Lapa in Bahia, and it fits this place with unnerving precision. Cathedrals spend centuries trying to produce the feeling that Terra Ronca produces for free. For most of the year the cave is a destination for people in helmets and neoprene. For that one week it belongs to families in flip-flops, singing.

Terra Ronca does not stand alone. It anchors the Terra Ronca State Park, some 57,000 hectares of limestone country around the municipality of Sao Domingos, which holds the largest concentration of caves in Brazil's centre-west, roughly two hundred of them catalogued and more still being surveyed. Only two are open to visitors on any ordinary day: Terra Ronca itself and Angelica. The rest are the business of speleologists and the small community of local guides who know which passages flood and when. That restraint is deliberate. These systems are alive, still being carved by rivers that swell violently in the wet season, and the same water that built the galleries can fill them in an afternoon.

Inside, the cave is less a tunnel than a building with several exits. The Claraboia is a skylight, a hole in the roof that drops a column of daylight into the dark and offers an early way out to anyone who has had enough. The Oco das Araras is another opening, and it comes with a soundtrack of its own: macaws and parrots nest in the rock around it, and their calls carry down into the gallery. There are further exits at the Pesqueiro, where the river runs shallow enough to fish. Walking Terra Ronca is therefore a strange kind of hike, half underground and half not, with the daylight arriving in vertical shafts and the river always somewhere nearby, still making the noise the cave was named for.