Free-field recordings of underground nuclear explosions constitute a unique data set within the near-source region of seismic events ranging in magnitude from M3 to M6.5. The term 'free-field' in this context refers to recordings from instruments emplaced in boreholes or tunnel walls such that the initial portions of the records (approx. 0.1 to 1 second) do not contain effects resulting from reflections at the free surface. In addition to the free-field instruments deployed to record ground motions from selected underground nuclear explosions at the Nevada Test Site (NTS) and elsewhere, surface arrays were routinely deployed to record surface accelerations and velocities from underground nuclear tests conducted at NTS. In this paper we provide an overview of the types of data recorded during the U.S. underground nuclear testing program and their availability, and briefly discuss potential uses of the data in investigating the limits on ground motions generated by earthquakes.