Almost every famous cave on Earth is dissolved out of limestone by acidic water, which is why they tend toward chambers, columns and dripstone. Aroe Jari is different. It is cut into siliciclastic sedimentary rock — sandstone, essentially compacted ancient beach and river sand — belonging to the Alto Garças and Vila Maria formations of the Rio Ivaí Group, laid down in the Paraná Basin during the Ordovician and Silurian, more than 400 million years ago. Sandstone does not dissolve; it erodes and collapses. The result is a cave that is extremely flat, a broad horizontal corridor 1,550 meters long rather than a stack of rooms, with waterfalls appearing along the way where surface streams find their way through the roof.

Near the entrance a stream has pooled into the feature that draws most visitors: the Lagoa Azul, the Blue Lagoon, a natural swimming pool of startlingly clear blue water. The colour is the point. Light entering the opening strikes the pool and throws it back onto the sandstone, so the walls of the cave carry a moving blue wash that has nothing to do with the rock's actual rust-and-cream banding. Swim in it and you are floating in the middle of the effect. It is cold, it is bright, and it lasts only as long as the sun sits at the right angle — a few hours in the middle of the day.

The first scientific accounts of Aroe Jari are credited to the caver Ramis Bucair, who explored it in the early 1970s. Formal protection followed slowly. In 1989 the cave was written into the Plano de Reordenamento da Ocupação da Chapada drawn up by FEMA, the state environmental foundation, and the following year federal decree 99.556/90 brought the surrounding area under environmental preservation. In that same year of 1989 the archaeologist Irmhild Wüst registered rock paintings at the site, formally recording as an archaeological place what the Bororo had treated as a sacred one for far longer. The cave carries the catalogue number MT-0038 and, from an earlier era of visitors, the alternative name Caverna do Francês — the Frenchman's Cave.

Aroe Jari sits 46 kilometers from the town of Chapada dos Guimarães, on the land of the Fazenda Água Fria in northern Mato Grosso, reached on dirt roads that discourage casual arrival. It is not the only cave in this sandstone: Caverna do Lago Azul and Caverna Kiogo Brado open in the same country, part of a system of passages the plateau has been quietly hollowing out of itself for a very long time. Visits are typically guided, and the walk through the full length of the cave takes hours. The Bororo, whose descendants still live in Mato Grosso, understood something about this place that no survey number captures: a kilometer and a half of level darkness under a plateau is a threshold, not a tourist attraction.