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Why they're dangerous: These huge South American snakes aren't venomous, but don't let that fool you. If a hungry anaconda takes an interest in you, you're probably not making it home for dinner. "They're ambush predators," says Wade. "You can't see them. They can strike very quickly. They've got a mouth full of backward-slanting teeth, and they'll throw coils around you and basically suffocate you. As you exhale, they'll just tighten, like a ratchet."

How to stay safe: If an anaconda gets its teeth into your arm, you'll have to push it into its mouth to free it from the backward teeth, then pull it out. "If you just pull," says Wade, "the teeth will go in deeper." And whatever you do, always carry a knife. "I talked to a fisherman who was attacked by one. He was grabbed on the upper arm, and the anaconda then climbed on the boat and started winding around him." The man managed to loosen the snake by sticking a knife between his body and the snake. "As it was tightening, it tightened itself onto the point of the knife, and that made it let go. Then he grabbed his machete and started hacking at it."