The LIGO detection of gravitational waves has opened a new window on the universe. I will discuss how the process of superradiance, combined with gravitational wave measurements, makes black holes into nature’s laboratories to search for new light bosons. When a bosonic particle’s Compton wavelength is comparable to the horizon size of a black hole, superradiance of these bosons into bound “Bohr orbitals” extracts energy and angular momentum from the black hole. The occupation number of the levels grows exponentially and the black hole spins down. For efficient superradiance of stellar black holes, the particle must be ultralight, with mass below 10^-10 eV; one candidate for such an ultralight boson is the QCD axion with decay constant above the GUT scale. Measurements of BH spins can disfavor or provide evidence for an ultralight axion. Axions transitioning between levels of the gravitational “atom” and annihilating to gravitons may produce thousands of monochromatic gravitational wave signals, turning LIGO into a particle detector.