More than 90% of the energy trapped on Earth by increasingly abundant greenhouse gases is absorbed by the ocean. Despite much progress over the past 20 years, monitoring the resulting ocean warming remains a challenging sampling problem. In this talk, I will present a new method that complements existing point measurements by inferring basin-scale deep-ocean temperature change from the travel times of sound waves that are generated by repeating earthquakes. I will show results from the East Indian Ocean and the Northwest Pacific Ocean. The measurements capture deep-ocean temperature variations on time scales from a few days to a decade. The approximate vertical structure of these variations can be inferred using measurements at different frequencies. I will argue that the new method, if extended globally, offers useful new constraints on the deep-ocean uptake and transport of heat.