Radio signals picked up by a NASA high-altitude balloon have been leaving scientists baffled. The balloon, part of NASA's experiment known as Antarctic Impulsive Transient Antenna, or ANITA, was floating 40 kilometres above the continent, in search of neutrinos and other particles when its sensitive radio antennas unexpectedly picked up signals that were coming from someplace below - way below - the Antarctic's frozen surface. Physicists say that for that kind of reception to occur the radio waves would have had to have penetrated 6,000 to 7,000 kilometres, or 3,700 to 4,300 miles, of solid ice and stone.
Although the balloon project has since been retired, researchers continue to study these unexplained transmissions and recently published their findings in the journal, Physical Review Letters. The researchers say that by all models of physics, the signals should have been absorbed by the rock and gone undetected.
Scientists know that these are not neutrinos - the particles that they had expected - but are still trying to narrow down what kind of radio signals they're dealing with. Meanwhile, with the ANITA project retired, the next instruments to have a go at the mystery will be on board a work in progress: A Pennsylvania State University team is building something bigger and, they hope, better: The Payload for Ultrahigh Energy Observation mission.