The golden lancehead (Bothrops insularis) is found only on this Brazilian island. Image: Nayeryouakim / Wikimedia Commons

There’s an island off the southeastern coast of Brazil so dangerous that the country has made it illegal to step foot on it.

Twenty-five miles from the mainland sits Ilha da Queimada Grande, an island overflowing with deadly pit vipers known as golden lanceheads (Bothrops insularis). The critically-endangered vipers, which aren’t found anywhere else, are among the most venomous snakes on the planet. One bite could kill a human in less than an hour.

Scientists say there are a shocking five snakes per square meter on the island, with somewhere between 2,000 and 4,000 individuals roaming the island at any given time.

So how did the snakes take over the island? One legend claims that pirates put the deadly reptiles there on purpose in an effort to protect buried treasure—but the reality isn’t quite as riveting.

Image: YouTube

The island, now appropriately dubbed “Snake Island,” was once part of Brazil’s mainland. Rising sea levels separated the landmassnearly 10,000 years ago, creating an island and trapping the snakes that existed there. Without any serious predators on land, the lanceheads were able to multiply rapidly, quickly taking over the small island.

But that’s not all. Because the serpents had only transient birds to dine upon, they had to ensure the birds died quickly enough to gobble them up before they could escape. The snakes with more toxic venom naturally had an advantage. Over time, the vipers evolved to possess an extremely potent venom that could kill nearly any bird quickly. In fact, golden lancehead venom — which can melt human flesh — is up to five times stronger than that of any snake on the mainland.

Most sane humans stay far away, but a select few researchers do venture out to the island (with government permission) in order to obtain the snake’s venom, which has showed promise for improving heart problems.