Brazilians quibble over whether there are five venomous golden lancehead snakes per square meter on Ilha da Queimada Grande or just one. Either way, it's the highest density of snakes on the planet—far too many to make the island, located around 90 miles off the São Paulo coast, safe for anyone except experts to visit.
The snakes evolved their legendary venom after the island was isolated from the mainland around 11,000 years ago. The lanceheads that were trapped on what became known as Snake Island flourished in the absence of predators but were soon starved for available prey. Since the spit of land serves as a stopover point for migratory birds, the snakes soon began hunting in the treetops. But to do so, they needed fast-acting venom. As Smithsonian explains:
Often, snakes stalk their prey, bite and wait for the venom to do its work before tracking the prey down again. But the golden lancehead vipers can't track the birds they bite—so instead they evolved incredibly potent and efficient venom three to five times stronger than any mainland snake's—capable of killing most prey (and melting human flesh) almost instantly.
After a series of horror stories emerged from the island—such as the last lighthouse keeper's family to live there being killed by snakes after leaving a window open—the Brazilian government began strictly controlling visits. No one is allowed to even stop at the island without express permission to do so. And even on approved visits, like to maintain the lighthouse (which has been automated since that family's deaths in the 1920s) or conduct scientific research, a doctor must be present.
But just because there are laws against visiting Ilha da Queimada Grande unsupervised doesn't mean there aren't people who go anyway. You would think the snakes would be sufficient deterrent but, in fact, they're the source of the attraction. The island is the only place in the world where the golden lanceheads live, and with strict regulations limiting research, the demand from scientists and animal collectors means that a single snake can fetch $10,000 to $30,000 on the black market.
[h/t Atlas Obscura]