“And roughly, if we warm up the ocean temperature by one degree, the sound speed change—it would be four meters per second. In the new study, however, Wu, Callies and their colleagues show they can use the sounds produced by earthquakes instead. In an earthquake, some vibrations bounce off the seafloor and turn into sound waves that get picked up by seismometers and underwater microphones. The researchers looked at the travel times of these sound waves for 2,000 pairs of earthquakes that occurred in the East Indian Ocean between 2005 and 2016. Each earthquake pair happened in the same place but at different times, allowing the researchers to measure how much the sound waves sped up.