Off the shore of Brazil, almost due south of the heart of São Paulo, is a Ilha de Queimada Grande. The island is untouched by human developers, and for very good reason. Researchers estimate that on the island live between one and five snakes per square meter.
That figure might not be so terrible if the snakes were, say, 2 inches long and nonvenomous. The snakes on Queimada Grande, however, are a unique species of pit viper, the golden lancehead. The lancehead genus of snakes is responsible for 90% of Brazilian snakebite-related fatalities. The golden lanceheads that occupy Snake Island grow to well over half a meter long, and they possess a powerful fast-acting poison that melts the flesh around their bites. Golden lanceheads are so dangerous that, with the exception of some scientific outfits, the Brazilian Navy has expressly forbidden anyone from landing on the island.
Update: As many readers are keen to point out, this story is almost surely wrong. Unless the island is covered with like two or three mice or catchable birds per acre.