Scientists studying radio signals from beneath Antarctica's ice sheet remain baffled by mysterious pulses that appear to violate the laws of physics, nearly a decade after their initial detection. The Antarctic Impulsive Transient Antenna experiment has recorded signals emerging from deep underground at angles that should render them impossible to detect, according to researchers at Penn State University who published their latest analysis this week.
The anomalous radio waves present a puzzle that could reshape understanding of fundamental physics or reveal previously unknown atmospheric effects in one of Earth's most remote regions.
The ANITA experiment, which uses balloon-mounted radio antennas floating 25 miles above Antarctica, detected the unusual pulses in 2016 and 201812. Unlike typical cosmic ray signals that bounce off the ice surface, these radio waves appeared to originate from 30 degrees below the horizon, suggesting they traveled through thousands of miles of solid rock to reach the detectors34.
"The radio waves that we detected were at really steep angles, like 30 degrees below the surface of the ice," said Stephanie Wissel, associate professor of physics at Penn State42. According to standard particle physics, such signals should have been absorbed by the rock during their journey, making detection impossible.
Wissel emphasized that the anomalies are "very likely not neutrinos," the elusive particles the experiment was designed to detect34. The team's calculations indicate the signals would have needed to traverse 6,000 to 7,000 kilometers of rock before reaching ANITA's instruments2.
The world's largest cosmic ray detector has effectively ruled out the most exotic explanations for the phenomenon. The Pierre Auger Observatory in Argentina, which monitored cosmic events for 15 years, found no evidence supporting theories involving unknown particles that could tunnel through Earth12.
After analyzing 7.6 million cosmic events, researchers detected only one candidate upward-moving shower, consistent with background noise rather than the exotic particle interactions some scientists had proposed1. According to a study published in Physical Review Letters, if the ANITA anomalies represented genuine upward-traveling cosmic rays, the Argentine facility should have detected between 8 and 59 similar events2.
The IceCube Neutrino Observatory at the South Pole has similarly found no corroborating evidence for the signals3.
Scientists are preparing to launch PUEO, a more sensitive successor to ANITA, in December 202512. The Payload for Ultrahigh Energy Observations will feature 96 antennas and improved detection capabilities designed to solve the lingering mystery.
"My guess is that some interesting radio propagation effect occurs near ice and also near the horizon that I don't fully understand," Wissel said1. "Right now, it's one of these long-standing mysteries."
