Signs of oxygen production on the ocean floor in a remote part of the Pacific first observed in 2013 convinced ocean scientist Andrew Sweetman that his monitoring equipment was faulty. That’s impossible, the scientist thought. Oxygen-producing species simply do not live at this depth, where there’s no light to allow photosynthesis. However, the same readings were observed on three subsequent voyages to the Clarion-Clipperton Zone, an abyssal plain as wide as the continental United States and punctuated by seamounts. Initially skeptical, Sweetman and colleagues couldn’t dismiss the data as erro…