Does the Earth ring like a bell?

We all know that earthquakes cause our planet to vibrate. But scientists have discovered a constant low-frequency humming even in the absence of quakes. It occurs at frequencies between 2.9 and 4.5 millihertz—so low that it is inaudible to human ears. And it’s everywhere: “The earth is ringing like a bell all the time,” says Spahr Webb, a marine seismologist at Columbia University. There is speculation that the phenomenon may be caused by the collision of ocean waves or movements in the atmosphere. The bottom line: We just don’t know what causes the resonance.