I will discuss how black holes can become nature's laboratories for ultralight axions. When a boson'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. Previously open parameter space of axions can be constrained by observations of rapidly spinning black holes. Such black hole-axion `gravitational atoms' can also source up to thousands of monochromatic gravitational wa