Scientists have recorded a strange sound deep underwater that they believe is being caused by the Earth itself, but they’re not sure what it is or where exactly it’s coming from. It sounds like an eerie hum, and unlocking the secret to why it is making this noise could mean unlocking the secret to the Earth itself.

Scientists know that the Earth regularly expands and contracts, which creates a hum that we can’t hear with our own ears. They think it’s possible that ocean waves could be the cause of this noise, but they don’t know for sure.

But it’s a very tough question to answer, because the oceans are so vast and cover two-thirds of the Earth’s surfae. It’s hard for scientists to get enough stations to cover the oceans as well as they do on land. This latest discovery, which was published in the journal Geophysical Research Letters, is significant in that it’s the first time the hum has been captured beneath the sea, as previous attempts had been blocked by earthquakes and other sounds in the ocean depths.