For nearly 30 years, researchers have understood that microseismic activity far too small to be felt by human beings is constantly producing a planetary “hum” on Earth that likewise can’t be heard without very sophisticated equipment.
This “free oscillation” and its source confounded these researchers ever since. Some believed it was caused by the ebb and flow of ocean waves, while others claimed it was due to the collision of ocean waves. But two years ago, a group of scientists determined it was both. But the “hum” had never been heard beneath the ocean’s surface, until now.
A new group of researchers traveled to La Reunion Island to the east of Madagascar in the Indian Ocean. Using free-fall seismometers, filters, noise reduction, and calculations over a period of 11 months, they were finally able to isolate the free oscillation from normal ocean noise levels and found “very clear peaks.”
The researchers found the hum resonates at a frequency about one ten-thousandth of the normal hearing level of human beings. They published their findings in the journal Geophysical Research Letters published late last month.