The open ocean is one of the most terrifying and unforgiving habitats on the planet. It is a brutal home, one that has shaped many of its inhabitants into formidable creatures. With so many dangerous animals lurking beneath the waves, a sort of biological arms race is in effect between predators and prey. After millions of years of evolution, there are now dozens of species of poisonous fish, sea snakes, and other ocean life that could seriously ruin a scuba diving vacation.