Natural disasters like volcanic eruptions occur infrequently, but if they occur near nuclear power plants or high‐level radioactive waste repositories, local and global communities can be threatened. Ideally, such facilities should be constructed only where geologic risk is very low. Estimating the probabilities of such events requires a comprehensive understanding of site geology and the geologic processes operating in the site region on timescales of 104 to 107 years. In light of these requirements, geologists and geophysicists must continually improve techniques for site characterization.