The Earth is singing, or at least humming, and scientists aren’t really sure why. However, a new study could lead to a breakthrough in understanding this bizarre phenomenon, and the answer to this riddle could lie within the ocean depths, experts believe.

This deep hum has been known about by scientists for quite a while, and researchers have theorized that it has something to do with how the Earth regularly expands and contracts. But a new study narrows down the source to the ocean waves.

The study, published in the journal Geophysical Research Letters, is significant because it is the first time scientists have successfully captured this hum under the ocean waves, which had been difficult to do in the past because of background noise from earthquakes and other ocean sounds. Scientists haven’t cracked the code yet, but this new breakthrough could help them get closer to the source of this mysterious noise.

“The Earth’s hum is the permanent free oscillations of the Earth recorded in the absence of earthquakes, at periods above 30 s,” reads the abstract of the paper. “We present the first observations of its fundamental spheroidal eigen modes on broadband ocean bottom seismometers (OBSs) in the Indian Ocean. At the ocean bottom, the effects of ocean infragravity waves (compliance) and seafloor currents (tilt) overshadow the hum. In our experiment, data are also affected by electronic glitches.”