Seventy years ago a rancher near Roswell, New Mexico, found some peculiar wreckage of silver foil and sticks. An official US army statement claimed it was a “flying disc”, leading to headlines that an alien flying saucer had been captured.

A second statement said the debris was just a weather balloon. The media accepted this version, although UFO enthusiasts still believe the Pentagon has a crashed extra-terrestrial spacecraft.

The second explanation was correct if misleading. It was an exotic weather balloon from a classified military programme, the existence of which was not revealed until 1994. The secret Project Mogul involved a cluster of meteorological balloons designed to carry microphones to a high altitude.

In the oceans there is a layer called the “deep sound channel”, which can carry sound for long distances, allowing submarines to be detected at long range. Scientists thought there might be a similar layer in the atmosphere which would channel loud noises – specifically Russian ballistic missile launches and nuclear tests. They calculated that such launches could be located from thousands of miles away.

Unfortunately the atmospheric sound channels, located in the 1960s, proved too high for the Mogul balloons. One channel was found at an altitude of 45-50 kilometres (28-31 miles), another at 80-90 kilometres (50-56 miles).

Raising instruments to these altitudes is impractical, but Swedish scientists have been using the sound channels for more than 30 years, catching reflections from them at ground stations. The equipment can track reflected infrasound from distant thunderstorms, meteors and volcanoes, and even pick up sonic booms from Concorde flights.

Source: The Guardian