14 DAY TRIAL // Volcanoes are usually courteous enough to announce that they are about to erupt by triggering a series of small earthquakes.
University of Washington scientists explain that these very small earthquakes can sometimes follow one another so quickly that they end up forming a harmonic tremor.
The human ear is not able to detect this harmonic tremor. However, researchers maintain that it exists and that it is strikingly similar to the noise produced by various musical instruments.
The only major difference is that it all happens at very low frequencies, Science News explains.
Specialists claim that, in 2009, Alaska's Redoubt Volcano produced such harmonic tremors before six different eruptions.
They say that, as pressure kept building up, the tremor switched to higher frequencies (i.e. it started at about 1 hertz and glided upward to roughly 30 hertz), and that it stopped almost instantaneously moments before the volcano erupted.