Mysterious rumbling noises of an unknown origin have been detected in the stratosphere of our planet Earth.
A batch of solar-powered hot air balloons originally designed to collect data on volcanoes were doing their routine data collection when they stumbled upon the mystery. Typically, microphones operating in this layer of Earth’s atmosphere hear ultra low-frequency reverberations from the Earth's surface, like echoes of thunder, ocean waves, or reverberations from man-made explosions, for example. But this time, the infrasound noises were of a quite puzzling nature.
“[In the stratosphere,] there are mysterious infrasound signals that occur a few times per hour on some flights, but the source of these is completely unknown,” said Daniel Bowman, a principal scientist at Sandia National Laboratories.
Bowman presented his research on Earth's stratospheric sounds at the 184th Meeting of the Acoustical Society of America in Chicago. In their line of study, the researchers resort to relatively inexpensive balloons equipped with microbarometers, which they release and then track with GPS.
"Our balloons are basically giant plastic bags with some charcoal dust on the inside to make them dark. We build them using painter’s plastic from the hardware store, shipping tape, and charcoal powder from pyrotechnic supply stores," Bowman explained.
As they are released, the Sun heats the air inside them, rendering them buoyant, and allowing them to drift to an altitude of over 20 km (66,000 ft.) During such forays, the contraptions recorded the low, recurring rumbles. As they ponder their source, the researchers say they will continue to investigate the stratosphere's sounds in different regions of the world, and during diverse seasons, hoping for clues.