Ilha da Queimada Grande is an island off the coast of Brazil that is more commonly known as "Snake Island." The British navy forbids visitors due to the extremely venomous snakes that live there. With 1-5 snakes per square meter, the island has the highest concentration of venomous snakes in the world.
The golden lancehead is a pit viper species that lives only on the island. Its venom is up to five times more potent than normal pit vipers living in mainland Brazil.
The snakes are described as moving landmines, but they actually spend most of their time in trees, hunting the migratory birds that are their primary food source. Researchers believe that the island was once connected to the mainland, but rising seas cut it off. The snakes then evolved their organ-liquefying venom so that their strikes would kill the birds before the birds flew away.
A lighthouse on the island used to be manned, but was automated in the 1920s. Local legend says the change was made after a family that tended the lighthouse in 1909 awoke to a snake crawling in through the window. The family attempted to flee but was attacked by snakes in tree branches and didn't make it.
For the few people who are allowed onto the island, the navy orders that a doctor be present in case an anti-venom needs to be administered. A researcher interviewed by Vice said it's still highly probable that the victim will die.
Despite the navy's attempts to keep people away, smugglers visit the island and steal the snakes which then make their way to buyers around the world. Other bio-pirates (actual term) bribe researchers and navy sailors to get snakes for them. The going rate for the snakes in 2014 was thought to be between $10,000 and $30,000 each.
Researchers are allowed to remove the snakes legally in order to investigate potential applications for the venom. Certain compounds in it have shown promise as drugs for heart disease, blood clots, and cancer.