Queimada Grande was not always an island. Thousands of years ago, during the last ice age, it was connected to the Brazilian mainland by a land bridge. When sea levels rose, the connection drowned, and whatever animals happened to be on the rock were stranded. For most species, isolation on a 430,000-square-meter island would mean extinction. For the ancestors of the golden lancehead, it meant transformation. Cut off from mainland prey -- rodents, lizards, the usual diet of a pit viper -- the snakes adapted to hunting the one reliable food source available: migratory birds. Over millennia, they developed venom fast enough to kill a bird before it could fly from a branch, and tails long enough to navigate the canopy where those birds perched. The island made the snake, and the snake came to define the island.

The island's name tells a story of human ambition defeated. Queimada means "forest fire" in Portuguese, and the name comes from historical attempts by locals to clear the land by burning, hoping to establish a banana plantation. The fires stripped portions of the island to bare rock, but no plantation ever took hold. What the fires did accomplish was to concentrate the snake population in the remaining rainforest, which today covers just 0.25 square kilometers of the island's total area. The rest is open grassland and exposed rock where birds rarely linger and snakes find little cover. A lighthouse was built in 1909 to warn ships away from the island's coastline -- a practical necessity given the rocky approaches, but also an unintentional symbol. Even the purpose of the island's one structure was to tell people to stay away.

Popular accounts once claimed the island held 430,000 snakes, a number that would mean roughly one snake per square meter of surface area. The first systematic population study told a different story. Researchers estimated 2,000 to 4,000 golden lanceheads, nearly all of them concentrated in the rainforest patch. The island also hosts a population of Dipsas albifrons, a non-venomous species, and 41 recorded bird species -- though the golden lancehead preys on only two: the southern house wren and the Chilean elaenia. This narrow dietary specialization, combined with the tiny range and a population too small to maintain genetic diversity, has earned the golden lancehead a critically endangered listing on the IUCN Red List. The snakes are simultaneously the island's greatest threat and its most vulnerable inhabitants.

In 1985, the Brazilian government created the Ilhas Queimada Pequena e Queimada Grande Area of Relevant Ecological Interest, encompassing 33 hectares of protected territory that includes the island and its smaller neighbor to the west. The Brazilian Navy enforces the closure. Only research teams with waivers from the Chico Mendes Institute for Biodiversity Conservation are permitted to land, and their visits are strictly regulated. The protection works in both directions: it keeps humans safe from one of the most concentrated populations of venomous snakes on the planet, and it keeps the snakes safe from the kind of disturbance -- habitat destruction, poaching, introduced species -- that has driven countless island endemics to extinction. Queimada Grande sits in the Atlantic, a low green silhouette against the horizon, close enough to the mainland to see on a clear day, and completely off-limits to anyone who might want to set foot on it.