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Scientists are trying to solve a decade-long mystery by determining the identity of anomalous signals detected from below ice in Antarctica.

The strange radio waves emerged during a search for another unusual phenomenon: high-energy cosmic particles known as neutrinos. Arriving at Earth from the far reaches of the cosmos, neutrinos are often called ghostly because they are extremely volatile, or vaporous, and can go through any kind of matter without changing.

Over the past decade, researchers have conducted multiple experiments using vast expanses of water and ice that are designed to search for neutrinos, which could shed light on mysterious cosmic rays, the most highly energetic particles in the universe. One of these projects was NASAs Antarctic Impulsive Transient Antenna, or ANITA, experiment, which flew balloons carrying instruments above Antarctica between 2006 and 2016.

It was during this hunt that ANITA picked up anomalous radio waves that didnt seem to be neutrinos.

The signals came from below the horizon, suggesting they had passed through thousands of miles of rock before reaching the detector. But the radio waves should have been absorbed by the rock. The ANITA team believed these anomalous signals could not be explained by the current understanding of particle physics.

Follow-up observations and analyses with other instruments, including one recently conducted by the Pierre Auger Observatory in Argentina, have not been able to find the same signals. The results of the Pierre Auger Collaboration were published in the journal Physical Review Letters in March.

The origin of the anomalous signals remains unclear, said study coauthor Stephanie Wissel, associate professor of physics, astronomy and astrophysics at the Pennsylvania State University.

Our new study indicates that such (signals) have not been seen by an experiment like the Pierre Auger Observatory, Wissel said. So, it does not indicate that there is new physics, but rather more information to add to the story.

Larger, more sensitive detectors may be able to solve the mystery, or ultimately prove whether the anomalous signals were a fluke, while continuing the search for enigmatic neutrinos and their sources, scientists say.

The search for neutrinos

Detecting neutrinos on Earth allows researchers to trace them back to their sources, which scientists believe are primarily cosmic rays that strike our planets atmosphere.