Scientists have found a link between the ’roundness’ distribution of tsunami deposits and how far tsunamis reach inland. They sampled the ’roundness’ of gravel from different tsunamis in Koyadori, Japan, and found a common, abrupt change in composition approximately 40% of the ‘inundation distance’ from the shoreline, regardless of tsunami magnitude. Estimates of ancient tsunami size from geological deposits may help inform effective disaster mitigation.