As we send telescopes into space to investigate the mysteries of galactic water, there's a surprising amount we don't know about our own terrestrial water. In this issue of Photonics Focus, we jump in, examining light science's impact on this research. From  underwater imaging instruments with unique requirements for pressure, refraction, and lighting, to giant clams whose light-harvesting abilities inspire new photonics research, and to fiber optic cables that crisscross the ocean floor, photonics may yet reveal unimagined mysteries in the deep.

The cover shows images of a variety of marine organisms taken with the benthic underwater microscope developed at Scripps Institution of Oceanography.