…from Wired Science…
“The once-mysterious planetary hum of the Earth is getting put to use by scientists mapping the planet’s interior.
“Ocean wave interactions, primarily along the Pacific coast of North America, generate a vibration with a frequency of about 10 millihertz, the background buzz of the globe.
“As the hum moves through the Earth’s crust, it speeds up and slows down in response to the different materials it moves through. Scientists know from many experiments tracking how earthquake waves move through the Earth that colder, denser materials tend to speed waves up and hotter ones tend to slow them down. By looking at those changes, a team led by Kiwamu Nishida of the University of Tokyo generated a map of the interior of the planet, as reported Thursday in the journal Science.”