THEY live underground. They are everywhere but seem to come from nowhere. They barely exist, but never leave. If sounds have shadows, they are the shadows of a sound.
Researchers call them the background free oscillations of the Earth. But last year, when a pair of Japanese geophysicists named Naoki Suda and Kazunari Nawa dredged them out of a mass of seismic data, some people called them a hum. That's a comforting thought: a mystic Om, perhaps, or just the warm, cosy sound of a planet going about its business.
Don't try to tune in, you'll never hear it, though. The Hum is far too low for human ears to detect and is so feeble that a single 5.5-magnitude earthquake can blot it out. That's just as well because, if you could hear it, the Hum might drive you mad.
"It's a very messy noise," says Hiroo Kanamori, a geophysicist ...
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