This is an analysis of how conditions in the oceanic environment differ from those in the familiar terrestrial world, describing the techniques and ingenuity required to reveal the populations inhabiting the colossal volume of the deep oceans and how its inhabitants have adapted to survive and flourish within it. Many issues of deep sea marine life are covered including: surface phytoplankton's roles; chemosynthesis at hydrothermal vents and cold seeps; reduction of biomass with depth and energy conservation; mechanoreception; chemoreception; vision; animal life styles, seasonability, sex and size; and the unique variety of life in the deep ocean both on the sea floor at in midwater.