Abstract : Black holes, particularly the supermassive black holes (SMBHs) in the centers of galaxies, are extraordinary laboratories for doing fundamental physics. Black holes provide, somewhat obviously, the opportunity to study the deepest gravitational potentials in the universe and strong-field General Relativity. But they offer so much more! As well as being potent accelerators of Standard Model particles, studies of SMBHs provide multiple pathways for exploring physics beyond the Standard Model, especially in the axion-sector. In this talk, I will explore these topics with particular emphasis on what can be learnt from existing X-ray observations of active galactic nuclei (AGN, i.e. accreting SMBHs). I will discuss how X-ray observations of AGN probe strong gravity and black hole spin which, in turn, inform how the SMBHs have grown over cosmic time. And I will discuss how studies of both SMBH super-radiance and AGN embedded in the magnetize intracluster medium of galaxy clusters has produced today’s leading constraints on the properties of ultralight axion-like-particles. I will conclude with a brief discussion of the future, including the UMD-led Advanced X-ray Imaging Satellite (AXIS) which, if selected by NASA this summer, will carry these studies into the 2030s.