Twenty years ago, somewhere above the Antarctic ice, a balloon-borne radio antenna recorded an event that, by every available standard of contemporary physics, should not have happened. Eight years after that, it happened again. A particle — or something that behaved like a particle — appeared to have travelled directly upward through approximately 6,000 kilometres of solid Earth, carrying enough energy to register as a brief, bright burst of radio waves at the antenna’s altitude of 37 kilometres above the ice. The physics of the situation said this was impossible. The instrument recorded it anyway. Two decades later, despite the existence of more than 40 published theoretical papers attempting to explain the two events, despite cross-checks against every other ultra-high-energy particle detector currently operating, despite the systematic ruling-out of the most obvious interpretations, no consensus exists in the particle physics community as to what the species’ instruments actually saw on those two December nights in 2006 and 2014. The leading remaining hypotheses include the existence of one or more particles that do not currently appear in the Standard Model. The Antarctic Impulsive Transient Antenna recorded the events. The events have not gone away.
According to Penn State University’s announcement of the June 2025 Pierre Auger Observatory analysis that ruled out the leading neutrino interpretation of the anomalies, ANITA was designed to detect ultra-high-energy cosmic neutrinos by listening for the radio bursts they produce when they interact with the Antarctic ice. Neutrinos are the most weakly interacting particles in the Standard Model — so weakly interacting that approximately 100 trillion of them pass through every square centimetre of the human body every second, with essentially none of them being stopped or absorbed along the way. At extremely high energies, however, the probability of a neutrino interacting with matter increases substantially, and ANITA was specifically calibrated to detect the radio signatures of these rare high-energy interactions. The instrument flew on four separate balloon missions between 2006 and 2017, each lasting roughly a month, with the balloon drifting in a circular pattern around the Antarctic continent at an altitude of 37 kilometres. Across the four flights, ANITA detected approximately 50 radio events that matched the expected signature of cosmic-ray air showers, and two events that did not.
What makes the two events anomalous
The standard signature of a downward-travelling cosmic ray, when its air shower’s radio emission is reflected off the surface of the Antarctic ice and detected by ANITA, includes a specific phase inversion that distinguishes a reflected signal from a direct upward-travelling one. The two anomalous events did not show this phase inversion. The signals appeared to be travelling directly upward from the ground, at very steep angles below the horizon, without having been reflected from the surface. Per CNN’s coverage of the broader scientific puzzle and the implications of the recent follow-up studies, the geometric implication is straightforward and serious. For a signal to arrive at ANITA’s altitude at 30 degrees below the horizon, the particle producing it would have had to travel through approximately 6,000 kilometres of solid Earth. At the energies involved — approximately 0.6 exa-electronvolts, or 0.6 × 10^18 eV, an enormous amount of energy for a single particle — no Standard Model particle is expected to survive that journey. Photons, electrons, muons, protons, and ordinary cosmic rays would all be absorbed by the Earth’s interior long before emerging on the other side. Even ordinary neutrinos, despite their weak interactions, are expected to be substantially absorbed at these energies after traversing this much rock. The two events, in essential terms, should not have been detectable at all.
The initial hypothesis — that the events were produced by tau neutrinos, a heavier flavour of neutrino that produces tau leptons when it interacts, and that the tau leptons subsequently decayed in the atmosphere to produce the observed radio signature — was an attempt to keep the explanation within the Standard Model. The hypothesis required that a substantial flux of ultra-high-energy tau neutrinos was reaching Earth from cosmic sources, that some of these were “Earth-skimming” through the planet, and that the tau leptons produced in the ice were decaying in such a way as to produce the upward radio signature ANITA detected. The hypothesis was testable. If it was correct, the IceCube Neutrino Observatory at the South Pole and the Pierre Auger Observatory in Argentina — both of which are substantially larger and more sensitive than ANITA — should have detected comparable signals during their overlapping observing periods. In March 2025, the Auger team published a paper in Physical Review Letters reporting that they had not. After 15 years of data collection across a 3,000 square kilometre detection area, the Pierre Auger Observatory had seen no events matching the ANITA anomaly signature. The tau-neutrino interpretation was, by the standard methodology of particle physics, ruled out.
What the explanations look like now
The exclusion of the tau-neutrino interpretation has, over the past year, substantially narrowed the field of possible explanations for the ANITA anomalies. The remaining hypotheses fall into two broad categories. The first is mundane: that the anomalies are not signals from new particles at all, but artefacts of some unusual interaction between conventional cosmic rays and the specific structure of the Antarctic ice sheet. A 2020 paper by the Virginia Tech physicist Ian Shoemaker and colleagues, published in Annals of Glaciology, proposed that the events could be explained by reflections of downward-travelling cosmic rays off compacted subsurface snow (firn) that lies beneath the surface ice, with the geometry of the firn layer producing an unusual unflipped phase signature. A 2023 paper proposed an alternative based on transition radiation produced when particles cross boundaries between materials of different refractive indices. Both explanations are still being investigated.
The second category of explanations involves new physics. As reported by Space.com’s coverage of the dark-matter and beyond-Standard-Model hypotheses for the ANITA anomalies, the leading new-physics candidates currently include: sterile neutrinos (a hypothetical fourth flavour of neutrino that interacts even more weakly than the three known flavours and could, in principle, traverse the Earth without being absorbed); supersymmetric particles, specifically the “stau” — the supersymmetric partner of the tau lepton, which would behave in roughly the way required to explain the events; dark matter particles decaying into Standard Model particles in the Earth’s interior; and magnetic monopoles, which would be heavy enough and weakly enough interacting to produce the observed signatures. None of these candidates has been independently observed in any other experiment. Each of them, if confirmed, would represent a Nobel-Prize-level discovery in particle physics.
What happens next
Per Physics World’s analysis of the broader theoretical landscape of the ANITA anomalies, the resolution of the puzzle will require either independent confirmation of the anomalous signal by a different detector, or a definitive demonstration that the signal can be reproduced by some conventional mechanism not previously considered. The successor experiment to ANITA — called PUEO, the Payload for Ultrahigh Energy Observations — is currently being designed and built by an international collaboration including Penn State, the University of Chicago, and the University of Hawaii. PUEO will fly on similar long-duration balloon missions over Antarctica, with substantially larger and more sensitive antennas than ANITA, and will be optimised specifically to either re-detect the anomalous signal type or to rule it out with high statistical confidence. The first PUEO flight is targeted for the late 2020s or early 2030s.
The interim status of the puzzle is unusual in contemporary physics. Most experimental anomalies, when they appear, are either rapidly confirmed by independent observations or rapidly ruled out by closer examination of the original data. The ANITA anomalies have, for nearly two decades now, occupied an intermediate position — not confirmed, not ruled out, with more than 40 published theoretical papers proposing various explanations and no clear consensus emerging from the analysis. The events themselves are extraordinarily rare; only two have ever been observed, across more than 100 days of cumulative observation time. Whether they were real signals from genuine particles, or rare instrumental artefacts that happen to look like signals from genuine particles, is a question the field has been unable to settle with the data currently available. The deeper question — whether the universe contains, beyond the 17 particles of the Standard Model, additional fundamental constituents that physicists have not yet identified — remains open. The two upward-going radio bursts that ANITA detected from the Antarctic ice may or may not turn out to have been the first experimental evidence that the answer is yes.
