Hydrothermal vents in the deep ocean are located at tectonic spreading ridges. While most of the water in the deep ocean is close to freezing, the water at hydrothermal vents is very hot and laden with chemicals.  In this extreme environment, certain species of Archaea and Eubacteria thrive, enabling a unique food chain including fish, shrimp, giant tubeworms, mussels, crabs, and clams.
Courtesy of NASA