The site covers roughly 250 square metres in total, but the main event is that single vertical panel. Carvers cut the figures in low relief - patient work in a hard metamorphic rock, done with stone tools and abrasive and time. Roughly fifteen metres of the panel carries the densest concentration of glyphs, at about head height. The imagery repeats: animals, fruits, human figures, and shapes that have resisted every attempt at classification. There is no accompanying script, no obvious narrative sequence, no comparable panel nearby to cross-reference. The carvings sit in the river as they have for centuries, submerged for part of the year and exposed for the rest, which is both what preserved them and what wears them down.

The honest archaeological answer is that nobody knows. Scholars attribute the panel to Indigenous people who lived in this part of Paraiba - the region was Potiguara country, and it was the Potiguara who supplied the term itacoatiara to Portuguese colonisers. Occupation of the area by Indigenous communities continued into the eighteenth century. But there is no securely dated context: no stratified excavation tying the carvings to a datable layer, no organic material fixed to the act of carving, no oral tradition recorded early enough to be treated as testimony. What survives is the surface. Attribution to the Indigenous peoples of the region is well founded; the specific culture, the century, and the purpose are not settled questions, and pretending otherwise does the carvers no favours.

In 1976 the Spanish engineer Francisco Pavia Alemany began a mathematical study of the panel, publishing his first results through the Instituto de Arqueologia Brasileira in 1986. He identified a series of ground-out hollows - he called them bowls - together with a petroglyph on the vertical face, arguing that the arrangement functioned as a solar calendar, with a gnomon casting the shadow of the first light of each day across the marks. He later turned to a scatter of small engraved signs, which he read as stars grouped into constellations; the panel is often said to depict Orion and the Milky Way. The Agrupacion Astronomica de la Safor summarised the work in 2005, and in 2006 the Egyptologist and archaeoastronomer Jose Lull included an essay on the Inga ensemble in a comparative volume on archaeoastronomy. It is a hypothesis, argued by people who did the measurements, and it remains one interpretation rather than a settled reading.

Undeciphered marks are magnets for explanation, and Inga has drawn its share of stories reaching well past anything the evidence will carry - the kind of theory that hands the credit for a South American monument to somebody from somewhere else. None of it holds up, and the pattern is worth naming for what it is rather than repeating. The archaeology is quieter and more interesting. People stood in this riverbed and spent an enormous amount of labour marking a rock face with a vocabulary of signs they clearly expected to be understood. That is a message written with confidence. The fact that the intended readers are gone is a loss, not a mystery requiring outside authorship.

Brazil's heritage service - then SPHAN, now IPHAN - listed the Inga Stone as a national monument on 30 November 1944, one of the earliest archaeological protections granted in the country. The Itacoatiaras of the Inga River also sit on Brazil's tentative list for UNESCO World Heritage status. Protection on paper has not stopped the damage. The site remains under constant threat from vandals and from people chipping at the panel, and every mark taken off it removes evidence that cannot be recovered. A carving that survived three hundred wet seasons in a river can be erased in an afternoon by someone with a chisel.