The atoll is built on the top of a seamount - a volcanic peak that rose from the ocean floor millions of years ago, reached the surface, and has been slowly sinking ever since. As the summit subsided, coral grew upward to stay in the sunlight near the surface, layer on layer, until a reef had built itself into the shape of an ellipse, about 3.7 kilometers on its east-west axis. The reef encloses a lagoon and two small islands - Ilha do Farol (Lighthouse Island, 3.5 hectares) and Ilha do Cemitério (Cemetery Island, 3.2 hectares). The highest point on the whole structure is just 3 meters above sea level. The reserve covers 35,186 hectares of reef, lagoon, and surrounding ocean. It lies 144 kilometers west of Fernando de Noronha and shares many of the same geological origins.

Count the seabirds in the air above Rocas at dusk and you begin to feel the scale of what lives here. BirdLife International has designated the atoll an Important Bird Area because of the extraordinary colonies it supports. The five dominant species are masked booby, brown booby, brown noddy, black noddy, and sooty tern. Red-billed and white-tailed tropicbirds nest on the rocks. Boobies dive for fish in the turquoise water over the reef, rising again with silver flashes in their beaks. Noddies and terns raise chicks in burrows or shallow scrapes on the beach. With no land predators, the atoll is one of the few places in the Atlantic where seabirds can reproduce at the density this habitat once supported everywhere. For every breeding pair you see, there are thousands more in the air.

Below the water, the story grows. Green sea turtles nest on the small beaches. Loggerheads and hawksbills feed in the lagoon. Lemon sharks patrol the channels. Land crabs of the genus Johngarthia scuttle over the sand, and the flat spray crab Percnon gibbesi clings to rocks in the intertidal zone. The corals themselves include Millepora alcicornis, the fire coral known locally as 'sea ginger' for the yellow-orange color of its branches. The water temperature stays remarkably steady, hovering between 27 and 28 degrees Celsius all year. Rainfall is low - 109 millimeters annually is the kind of number that would define a desert if this were not the open sea - and the islands stay hot, bright, and salt-scoured almost constantly. In 2001, UNESCO inscribed Atol das Rocas along with Fernando de Noronha as a joint World Heritage Site.

The Biological Reserve was created by decree on June 5, 1979. Under the IUCN classification system it is a Category Ia strict nature reserve, the most restrictive designation possible - administered today by the Chico Mendes Institute for Biodiversity Conservation (ICMBio). The reserve's stated objective is the full preservation of biota and natural processes, with human activity limited to what is necessary for ecological recovery and scientific study. Tourism is prohibited. Fishing is prohibited. Even research visits require specific authorization and strict protocols. A small team of ICMBio staff maintains a research station on Ilha do Farol, conducting long-term monitoring of turtles, seabirds, and reef health. Ships passing through must keep their distance. In an ocean transformed by industrial fishing and climate change, Rocas is the rarest thing of all: a reef left alone long enough to function as it was meant to.

The lighthouse on Ilha do Farol - a slender iron structure first lit in the 19th century and replaced several times since - marks the atoll for the ships of the transatlantic lanes. Standing beside it at dawn, you see the ring of reef catch the first light before the rest of the world: a bright crescent of breakers, white sand islands, and a lagoon that shades from pale green to electric turquoise as the sun climbs. The researchers who rotate through Rocas describe the same sensation in interviews over the years - the sense that this is what the Atlantic once looked like everywhere along this coast, a glimpse of what the sea can become when humans step back. For the boobies and the turtles and the sea ginger, it is simply home. For everyone else, it is a reminder of what protection can preserve.