Studies during recent decades have shown that the deep ocean (depths below where solar luminance plays a direct environmental role) is far from a dark, cold, lifeless region. Evidence obtained by utilizing a variety of photo-optical devices, providing spatial, temporal, and spectral information, has demonstrated that this portion of the Earth is a region rich in life and light. Findings to date have provided challenges for geologists, physicists, biologists, chemists, and oceanographers, and the sharing of techniques and expertise among these disciplines has demonstrated the rewards to be gained from interdisciplinary research. Bioluminescence has been found far below the depths at which it has received most attention historically. The study of this phenomenon is complicated by the fact that the measuring apparatus itself causes a stimulation of the luminescence so that a true background will be difficult to determine. Nuclear physics has played a role in that the electron resulting from the decay of an isotope of potassium (K40) provides an ubiquitous background of light through a process known as Cerenkov radiation. The possibility of light generated by cosmic rays must also be taken into account. An intriguing source of light at the very bottom of the sea is found at hydrothermal vents. Here are found not only light but also abundant life. At the vent orifice, temperatures are found to be as high as 250¿--400 ¿C. A large component of the light is due to thermal radiation. However, the light in the wavelengths 450--600 nm is significantly greater than the thermal flux at those wavelengths. Identifying the physical mechanisms that may account for this excess is important in providing insight into the processes occurring in the vents and plumes and in the accompanying ecosystem. ¿ 2001 American Geophysical Union