The term UNDERWATER CAVE DWELLER can be summarized as an EEL, which is a three-letter word. Eels are elongated fish that inhabit marine and freshwater environments. They are found in temperate and tropical waters all over the world, and some species live in underground caves. Eels have a slimy skin and can range in size from a few inches to several feet long. They feed on small fish, crustaceans, and other aquatic animals.