Brazilian wildlife photographer João Marcos Rosa has what are called strong nerves. He has climbed 35-metre-high giant trees in the Amazon basin to photograph harpies. They are among the largest birds of prey on earth. He was in Colombia in FARC rebel territory to track the last Andean bears. And he has travelled to photograph mountain gorillas on the border between Uganda and the Democratic Republic of Congo. But even for Rosa, this was something different: “These were the most complicated expeditions I’ve ever done – a 1.5-kilometre-long island in the middle of the Atlantic Ocean, no source of fresh water, and thousands of poisonous snakes.”

Rosa can nevertheless consider himself lucky. He is one of the few people who have ever set foot on Ilha da Queimada Grande. The island, 32 kilometres off the southern coast of Brazil, is a forbidden place, accessible only to the Brazilian military and a few selected biologists. No mammals, let alone humans, live on the 200-metre-high granite rock in the middle of the Atlantic. Even the lighthouse keepers, who once eked out a solitary existence here, left the island decades ago when the beacon was automated.

The rock rises abruptly from the water. The rough sea and the complicated landing make for quasi-perpetual isolation. Rosa has taken part in four expeditions to Queimada Grande: “There were difficulties on three of them,” he says, “and on one we couldn’t land at all because of the bad weather.” Those who want to reach the island face two main problems: On the beach of the coastal town of Itanhaém, 100 kilometres south of São Paulo and barely two hours away by boat, a strong wind blows most of the year, making disembarkation difficult. And then there is the landing. “The sea has to be extremely calm because there are no beaches on Queimada Grande. You can only get to the island over rocks,” says the 43-year-old. That is dangerous, especially with so much equipment. But that is by no means all, because the island hides a secret, which is why the military visit less regularly, and biologists very regularly: Queimada Grande is home to a unique population of Bothrops insularis, a highly poisonous lance viper that can only be found here.

In the Atlas of Superlatives, which lists the “biggest, fastest, longest, hottest, strongest, most gigantic and most dangerous record holders in the world”, it says about Ilha da Queimada Grande: “Whoever goes ashore risks his life”. And that is exactly why the island has attracted scientists for decades. “With about 45 snakes per hectare, Queimada Grande has the second densest snake population in the world,” says Marcus Augusto Buononato. “This is only surpassed by Shedao Island in northeast China.” Buononato should know, he worked for 13 years at the government-run Butantan Institute in São Paulo, which researches what substances from nature medicine can harness, and took part in numerous expeditions to the island. According to Buononato, about 2,000 snakes are said to live on the island, which covers only 43 hectares, or 0.43 square kilometres. According to other studies, there could be as many as 4,000.

But how did the animals manage to get here without being able to swim? “The snakes arrived at Queimada Grande about 11,000 years ago during the last ice age, when the island was still connected to the mainland,” says herpetologist Breno Damasceno, who has been studying the animals for 28 years. “When the ice age ended, the sea rose and what was once a mountain became an island. Since then, the snakes have been separated from their relatives on the mainland.”