Unexplained sounds detected in Earth's stratosphere perplex scientists

Scientists from Sandia National Laboratories launched solar-powered balloons carrying microphones to the stratosphere, a region of Earth's atmosphere around 31 miles above the planet. The microphones detected strange sounds that repeat a few times per hour, which have yet to be identified. The sounds were recorded in the infrasound range, meaning they were at frequencies of 20 hertz and lower, well below the range of the human ear. The team's research will be presented at the 184th Meeting of the Acoustical Society of America.