There are snakes that can kill you with a single bite and scorpions that can take you out with just one sting. But none of them have venom with the killing power of a snail. The geography cone snail (Conus geographus) has a toxin so powerful that a lethal dose for a human is just 0.029-0.038 milligrams.
There have been only 36 recorded human fatalities from it since 1670. That's because the geography cone snail lives in the ocean. But its unique venom delivery system is capable of penetrating the wetsuits of any humans foolish enough to get too close. BBC Earth reports:
To take down their targets, cone snails have modified teeth called "radulae". They are sharp, hooked and hollow, like a cross between a harpoon and a hypodermic needle. The snail launches one at an unsuspecting fish, whereupon it delivers a cocktail of toxins that target the nervous system. Once the fish is paralysed, the cone snail can devour it.