Project Mogul

Project Mogul (sometimes referred to as Operation Mogul) was a top secret project by the US Army Air Forces involving microphones flown on high-altitude balloons, whose primary purpose was long-distance detection of sound waves generated by Soviet atomic bomb tests.

[1] The project was moderately successful, but was very expensive and was superseded by a network of seismic detectors and air sampling for fallout, which were cheaper, more reliable, and easier to deploy and operate.

The project involved arrays of balloons carrying disc microphones and radio transmitters to relay the signals to the ground.

... To the untrained eye, the reflectors looked extremely odd, a geometrical hash of lightweight sticks and sharp angles made of metal foil.

"[4] Implementation of Mogul's experimental infrasound detection of nuclear tests exist today in ground-based detectors, part of so-called Geophysical MASINT (Measurement And Signal INTelligence).