I will discuss how black holes can become nature's laboratories for new ultralight particles and ongoing observations of gravitational waves can inform models of particle physics. When a particle's Compton wavelength is comparable to the horizon size of a black hole, energy and angular momentum from the black hole are converted into exponentially growing clouds of bosons, creating a gravitational atom in the sky. Theories beyond the Standard Model often include new, light, feebly interacting particles -- including the QCD axion -- whose discovery requires novel observations and search strategies. Previously open parameter space of axions can be constrained by observations of rapidly spinning black holes. Such `gravitational atoms' can also source up to thousands of monochromatic gravitational wave signals; searches are underway in current LIGO data, enabling gravitational wave detectors to discover or exclude new particles. If the axions interact with one another, instead of gravitational waves, black holes populate the universe with axion waves that may be detectable in dark matter searches.
