WHAT WE KNOW
When seismic instruments listen for earthquakes, they also pick up vibrations from storms moving over the planet's surface and waves crashing along the world's coastlines. This generates Earth's constant, distinct seismic hum.
WHAT WE DON'T KNOW
There remain some mystery sounds, including a rhythmic buzz emanating from the Gulf of Guinea in the Atlantic Ocean, the sources of which have not been identified.
Five amazing, clean technologies that will set us free, in this month's energy-focused issue. Also: how to build a better bomb detector, the robotic toys that are raising your children, a human catapult, the world's smallest arcade, and much more.