The majority of the Earth's habitable environments are physically located in environments that do not receive sunlight. Indeed, the largest potential habitats on Earth are located in the ocean, which covers approximately 70% of the Earth's surface (Fig. 1). The ocean's average depth is 4,000 m and reaches as deep as 11,000 m in the Marianas Trench. Depending on the quantity of particulate matter in the water, significant levels of sunlight—the driver of photosynthesis—penetrate only a few tens to hundreds of meters (300 m maximum) into the water. Thus, from a quantitative perspective, most of the ocean—roughly 1.27 × 1018 m3—exists in the dark, with a minor volume (3.0 × 1016 m3) actually penetrated by sunlight (Table 1). Considering that other habitats exist beneath the ocean water column, such as marine sediments, oceanic crust, and hydrothermal vents (Table 1), these dark ocean environments (Fig. 2 to to5)5) together comprise the largest collection of habitats by volume that life—in particular, microbial life—can occupy on Earth.