Optical atomic clocks have made huge improvements in precision and accuracy in the last decade, and led to great advances in time & frequency metrology. However, clocks are good for more than just keeping time. In this talk, I will explain how optical clocks can be used to observe low-frequency gravitational waves, in a part of the gravitational spectrum where strange new astrophysical sources may lurk. I will discuss our ongoing efforts to develop compact and portable optical atomic clocks, aimed at the (eventual) realization of such low-frequency gravitational wave detectors.