Boiúna (also called Cobra Grande, 'the Great Serpent') is the paramount aquatic cryptid of the Brazilian Amazon, reported across the basin from the Rio Negro and Solimões confluence at Manaus south to the Madeira and west into Peruvian Amazonia. Indigenous Tupi, Kokama, and Tikuna traditions describe Boiúna as an enormous dark serpent, thirty to 100 metres long, which lives in the deepest bends of the rivers and surfaces at night with two red eyes that ribeirinhos (river-dwellers) regularly mistake for the running lights of a motor boat. Boiúna is said to be capable of capsizing canoes and passenger ferries by surfacing beneath them, and is credited with a long catalogue of unexplained river losses.
The modern European catalog of reports dates from the 1820s. Portuguese colonial officials documented multiple Boiúna accounts at Óbidos and Santarém. In 1977, a commercial-fishing crew working the Rio Negro's Anavilhanas archipelago reported observing a dark shape 'longer than their thirty-metre boat' swimming parallel to their course at dusk. In 1999, a passenger ferry operator near Itacoatiara reported losing radio contact for ninety seconds during which the vessel was 'pushed sideways' by something beneath the hull; the subsequent port inspection noted unusual gouging on the keel. In 2011, a Manaus TV station's recording of a dark serpentine shape moving through the Negro at dusk circulated widely on Brazilian television without clear alternative identification.
Biologists have proposed the green anaconda (Eunectes murinus), which confirmed specimens have been measured to 5 metres and reliable reports to 8.8 metres; exceptionally large individuals in remote areas may exceed 10 metres. Boiúna's reported size far exceeds this, but the tradition's consistency across unconnected Indigenous peoples over centuries suggests a genuine observational core. Whether the Boiúna represents a folk-amplified anaconda, a species of giant reclusive fish (the Amazon's pirarucu can exceed 3 metres), or an as-yet-uncatalogued reptile, the Great Serpent remains the defining cryptid of the Amazon river system.