Even in the absence of giant earthquakes, this planet is still emitting a quiet hum. Researchers have measured it at the ocean’s depths. Scientists have been aware of this quiet hum for decades, and many seismic stations have measured it on the planet’s surface. No one has measured it underwater during periods without earthquakes, though—there’s been far too much background noise. Now, a team of European scientists has made that measurement using ocean-bottom seismometers.