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Off the shore of São Paulo lies an island forbidden to visitors, by order of the Brazilian Navy. They have a good reason: Ilha Queimada Grande, "Snake Island," is covered with deadly Golden Lancehead vipers.

Unique to the island, this species of serpent is believed to have the fastest acting venom of any Lancehead viper. The effects of its bite include bleeding orifices, brain hemorrhaging, and kidney failure.

The density of snakes on the island is estimated at one per square meter, many of which are found overhead, hanging from the trees. In other words: death, every 3 feet, in all directions. Locals in the coastal towns near the Queimada Grande happily recount grisly tales about the island. In one, a fisherman unwittingly wanders onto the island to pick bananas and is bitten only moments after stepping ashore. The fisherman manages to stumble back to his boat before dying in a pool of his own blood. 

More on Snake Island can be found on Atlas Obscura.

Islands to avoid:

• A World War II Anthrax test site in Scotland's waters
• One of Stalin's "special settlements" in Siberia, Nazino Island claimed the lives of thousands 
• The Andaman Islands are home to the most isolated tribe on Earth