Wilson next expands his argument about Earth’s incredible and little-known biodiversity from land to the marine world. The human ecological footprint is growing, as evidenced by warming sea temperatures, the ocean becoming more acidic due to increased carbon dioxide absorption, increased coral reef bleaching, and overfishing. These human-driven changes have reduced marine species’ populations and reduced their migratory routes. Despite these changes, “most of marine biodiversity persists” (114), but it is little known to scientists.

To emphasize that marine biodiversity is not well understood by scientists, Wilson takes readers on a journey from the surf zone to offshore coral reefs to the open water and then drops down first to “skim the surface” and finally into the deepest parts of the ocean (known as the abyssal benthos). Throughout this journey, he reiterates that there is life everywhere and that much of it is “barely visible if at all to the naked eye” (114). For example, the surf zone is teeming with meiofauna, or small invertebrates that live in the spaces between individual sand grains in both marine and freshwater environments. Little is known about these organisms, except that they occupy one of the most dynamic environments on the planet.

Wilson also notes that marine insects are rare and only found on the surface, which, as an entomologist, he finds surprising.