The reef system off northern Brazil was the Amazon Reef.
The discovery became widely known through a study published in Science Advances on 22 April 2016. The paper, “An extensive reef system at the Amazon River mouth,” described a large carbonate ecosystem on the continental shelf between the Brazil–French Guiana border and Maranhão State.
Researchers had investigated the region in 2012, following earlier clues from reef-fish observations and a 1977 paper with a hand-drawn map. Sampling revealed corals, sponges, rhodolith beds and other reef-associated organisms. The system covers roughly 9,500 square kilometres and stretches for about 1,000 kilometres.
The name can be misleading: the Amazon Reef is not a coral reef growing in the river’s freshwater channel. It lies offshore in the Atlantic, beneath or near the influence of the Amazon’s enormous sediment-rich plume. That murky environment limits sunlight and helps produce an unusual ecosystem in which sponges and other filter-feeding organisms are especially important.