What on Earth was that sound? Was it a bird? A plane? A humpbacked whale?

No, it was fiercely hot gas whooshing through the guts of a volcano — Arenal Volcano in Costa Rica, to be precise. Milton Garces, the director of the Infrasound Laboratory at the University of Hawaii, Manoa, recorded the sound clip we posted last week.

Garces explains the phenomenon this way:

NASA satellites have got Earth's volcanoes covered from orbit. They provide round-the-clock monitoring of volcanic eruptions in progress or those possibly on the way. Just two of the satellites used to monitor volcanic activity are Aqua (with its MODIS instrument) and Terra (with its ASTER instrument).

Volcanologists use the satellite data from NASA's fleet to detect heat and telltale volcanic gases emanating from volcanic vents. Also, Global Positioning System satellite devices allow researchers to gauge subtle changes in the land surface near volcanoes.

And once a volcano pops off, NASA satellites track drifting clouds that could threaten aircraft. (You may recall the shenanigans of a certain unpronounceable Icelandic volcano, Eyjafjallajökull, earlier this year.)

As the NASA birds pass silently overhead, Garces clambers up live volcanoes to record their subterranean rumblings. He uses the sounds to diagnose the physical status of volcanic plumbing systems – for example, whether they might be recharging with molten rock (magma) and getting ready to erupt.

These very low frequency waves are called infrasound. In fact, they are too low in their raw form to be audible to humans. So Garces speeds them up artificially to a frequency range the human hearing can detect. Here's how he explained it to us:

In other words, volcano infrasound is pretty interesting to a scientist, but you can't dance to it — or at least it would be the ultimate slow dance.