Using soundwaves cast from seafloor earthquakes, researchers demonstrate a new take on a largely abandoned way to measure ocean warming worldwide - seismic ocean thermometry. The method, which they tested in the East Indian Ocean and where they uncovered a decadal warming trend that exceeds previous estimates, has great promise to expand our ability to observe the rates and patterns of ocean warming and its effects on climate change, the study's authors show. While monitoring ocean warming is crucial to understanding and predicting future climate change, it remains a challenging phenomenon to quantify and is limited by a relatively small number of single-point measurements that sample a fraction of the vast and fathomless depths. Wenbo Wu and colleagues introduce seismic ocean thermometry, which uses low-frequency soundwaves from repeating undersea earthquakes to determine ocean temperature at far-larger scales. The speed of sound in seawater depends on its temperature. Thus, changes in the travel time of soundwaves between a source and receiver can infer average ocean temperature across great distances and at depth. While these principles were first explored as a way to monitor ocean temperatures more than 40 years ago, prohibitive expense and a concern for the impact of artificial sound sources on marine mammals led to abandonment of the approach. Here, Wu et al. resurrect this technique and demonstrate that it can be done passively, using natural seafloor earthquakes as a low-frequency sound source. To infer changes in temperature, the authors collected and analyzed data on 2,047 pairs of "repeating" earthquakes that occurred between December 2004 and June 2016 in the East Indian Ocean. They used the data to compile a marine basin-wide temperature profile spanning 3,000 kilometers. The results suggest a decadal warming trend that significantly exceeds previous estimates. "Wu et al. demonstrate how an intriguing combination of physical oceanography and classical seismological techniques potentially opens the way for an entirely new and globally capable observation system," writes Carl Wunsch in a related Perspective.