A concentration of late fourth- and early fifth-century sources seem to suggest that a massive earthquake shook the eastern Mediterranean in the second half of the fourth century CE, precipitating a tsunami that reached as far as Croatia, Northwestern Greece, Libya, and Egypt. This earthquake is conventionally dated to the morning of July 21, 365 CE. However, this neat picture of a single, universally-destructive event is open to question, for it is difficult to resolve the textual, archaeological, and geological evidence for seismological activity in the second half of the fourth century into a single, coherent picture. This Great Lecture uses that data, instead, to explore late Roman society’s ‘culture of risk’—its strategies for understanding, mitigating, and exploiting the manifold uncertainties of the physical and metaphysical world.