Atol das Rocas is a small reef system in the equatorial South Atlantic, about 266 kilometres off the coast of Rio Grande do Norte. It is the only atoll in the South Atlantic, a distinction that makes this remote formation scientifically important as well as visually unusual. Unlike a volcanic island with a high central peak, an atoll is built around a reef ring and lagoon. Rocas’s exposed land is low and limited, so tides, waves and seabirds shape its visible form from one part of the day to another.
The botanical garden’s protected-area catalogue describes an elliptical atoll with a reef ring of roughly 5.5 square kilometres and a lagoon concentrated toward the northeast. At low tide, parts of the central sandy area emerge; at high tide, water covers them again. Two biogenic islands, Farol and Cemitério, are formed from fragments produced by marine organisms, rather than being ordinary continental land. The reef itself includes crust-forming calcareous algae, vermetid gastropods, foraminifera and corals. The result is a compact but complex marine landscape.