The strange sound that comes from the depths of Antarctica and that science doesn't know what it is

Between 2016 and 2018, a disconcerting phenomenon for science was captured by a cosmic particle detector floating over Antarctica.

The Antarctic Impulse Transient Antenna (ANITA), an array of antennas carried by balloons at an altitude of 40 kilometers, recorded radio signals that, instead of reflecting off the ice as usual, came from below the horizon. That is, they passed through the Antarctic ice, with an orientation incompatible with the known principles of particle physics.

After nearly a decade of obtaining these results, it is believed that this could indicate the existence of new physics or completely unknown particles, according to a study published in Physical Review Letters, which uses 15 years of cosmic data from the Pierre Auger Observatory in Argentina.

"The radio waves we detected were at really steep angles, about 30 degrees below the ice surface ," explains Stephanie Wissel, associate professor of physics, astronomy, and astrophysics at Pennsylvania State University and a researcher on the ANITA team.

This seemingly technical detail hides an impossibility: to reach ANITA in that orientation, the signals had to pass through between 6,000 and 7,000 kilometers of solid rock. Any radio waves should have been completely absorbed during that underground journey, rendering them undetectable. Yet there they were, clear and powerful.

"It's an interesting problem because we still don't have an explanation for what these anomalies are," Wissel admits in a press release.

The ANITA detector searches for neutrinos, almost ethereal particles known as "ghost particles" for their ability to pass through matter with little interaction.

Neutrinos, in turn, are extraordinary messengers of the universe. Born in violent cosmic events like supernovae or even the Big Bang, they travel through the universe at speeds approaching the speed of light, passing through planets like ghosts. According to Wissel, " there are a billion neutrinos passing through your thumbnail at any given moment."

The mystery is that ANITA floats over Antarctica because this frozen continent is a perfect laboratory, far from interference and with vast expanses of ice that act as a natural detector. Initially, suspicion naturally fell on tau neutrinos, which, when they collide with ice, can generate radio emissions called "ice showers" and produce a secondary particle—the tau lepton—that quickly decays into an "air shower."

These types of events can be analyzed to identify their origin, just as a ball's trajectory can be deduced from the angle at which it bounces. But in this case, the angle was so unusual that the same principle couldn't be applied.

What's disconcerting is that the anomalous signals weren't picked up by other detectors. Neither the prestigious IceCube Experiment nor the Pierre Auger Observatory detected the upward showers of air that should accompany events of such magnitude.

The researchers have ruled out all conventional explanations. The signals show "strong horizontal polarization, but without the polarity reversal expected for reflected pulses," characteristics that do not fit with any known phenomenon.

Consequently, the scientific community is asking: Were they neutrinos with unknown properties? New interactions? Or something entirely different, like dark matter, that invisible form of matter that makes up 85% of the universe but that we have yet to be able to directly detect?

However, the new study's analyses seem to dampen that hypothesis, at least for now. Wissel comments: "My guess is that there's some interesting radio propagation effect occurring near the ice and also near the horizon that I don't fully understand."

The answer could come with PUEO (Payload for Ultrahigh Energy Observations), the successor to ANITA currently under development. This new detector will be larger and more sensitive, capable of detecting weaker signals and, scientists hope, finally solving the mystery.

"I'm excited that when we fly PUEO, we'll have improved sensitivity," Wissel explains. "In principle, we should detect more anomalies, and perhaps we'll come to understand what they are."

Meanwhile, the anomalous signals from Antarctica are adding to the list of unsolved scientific enigmas. As Wissel acknowledges, " right now, it's one of those long-standing mysteries " that's keeping physicists around the world on their toes.