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A parked truck with the engine running. That's the sound some people have compared to the mysterious, non-stop noise which is emanating from our home planet. We've been aware of this phenomenon for decades now, and while the source of the commotion remains unknown, the scientists who study it have just made an important breakthrough. Here's a quick history lesson. In the 19th century, geologists began to suspect that the earth might be producing a constant hum, one which rings out even in the absence of earthquakes and seismic events. They also reasoned that the noise must be too quiet for our human eardrums to hear. The official name for this drone is "permanent free oscillations." Until somewhat recently, its existence was only theoretical. A team led by seismologist Hugo Benioff did try to detect the signal in 1959. But their efforts failed because, at the time, science did not yet possess any instruments that were sensitive enough to pick up the hum. Theory became fact with the advance of technology. In 1997, scientists at the Showa Station — a Japanese research base in eastern Antarctica — were finally able to prove that permanent free oscillations really do exist. The good news was announced a year later, when the Showa team published their findings. Since then, numerous other teams have observed the same noise. Now, for the first time ever, the earth's hum has been recorded using seismic equipment on the ocean floor. This is a big deal because every previous study which has documented the noise did so with land-based instruments. The achievement was a hard-won prize. Martha Deen is a geophysicist with the Institut de Physique du Globe de Paris ("Paris Institute of Earth Physics"). Under her leadership, an international team reviewed data collected over an 11-month period from 57 seismometer stations on the Indian Ocean's floor. And that was just the first step. Next, the researchers eliminated all forms of audio interference — such as water currents and technical glitches — from the recordings made at two of the stations.