Approximately 70% of the surface of Earth is the bottom of the seafloor and about 80% of the seafloor is the deep seafloor (at deeper than 200m below the sea-surface). This book discusses the deep sea. It provides topics on the biodiversity, human dimension and ecological significance of the deep sea, including the metagenomic and metatranscriptomic analysis discoveries; the biodiversity and life-history strategies of deep sea megafauna in the Mediterranean Sea; and prokaryotes in metal deposits on the deep seafloor.