VF-041 · COSMIC · OPENThe ANITA Anomalous Events
Twice, a NASA balloon over Antarctica recorded a radio pulse coming up out of the ice from a direction nothing should be able to reach.
2006 · Antarctica; ANITA long-duration balloon flights over the Antarctic ice sheet · 12 min read
The Hook
The ANITA payload hangs beneath a helium balloon roughly thirty-seven kilometres above Antarctica, its antennas pointed down at the ice. It was built to listen for a particular kind of radio flash: the signature of an ultra-high-energy neutrino striking the ice sheet. In practice its most common catch is something else — ordinary cosmic rays, which produce air showers in the atmosphere and whose radio emission bounces off the ice surface and back up to the instrument.
A radio wave that reflects off the ice comes back phase-inverted. It arrives upside down. Across four flights ANITA has logged 71 cosmic-ray-consistent events, and 64 of them show exactly that inversion. It is the signature of a bounce.
Twice, it did not.
On 28 December 2006, and again on 20 December 2014, ANITA recorded a pulse that arrived from steeply below the horizon — 27.4 degrees below on the first occasion, 35.0 degrees below on the second — with no polarity inversion at all. The waveform looked like a signal that had come up through the ice and travelled directly to the payload, not one that had struck a mirror.
To arrive from those angles, whatever produced the signal had to have crossed roughly 5,740 kilometres of Earth in the first case and 7,210 kilometres in the second. At the energies involved — a few tenths of an exa-electronvolt — that is eight to ten neutrino interaction lengths of rock. Nothing in the Standard Model of particle physics gets through that.
Eighteen years later, nobody has explained either event.
The Facts
ANITA — the Antarctic Impulsive Transient Antenna — is a NASA-sponsored long-duration balloon payload led by the University of Hawai'i at Mānoa, with collaborators including UCLA, Ohio State, Delaware, Kansas, Washington University in St. Louis, University College London, the University of Chicago, Penn State, National Taiwan University and NASA's Jet Propulsion Laboratory. It has flown four times: 2006–07, 2008–09, December 2014 and December 2016, each time on a circumpolar course over the Antarctic ice.
Its design purpose is to detect cosmogenic neutrinos at around 10^18 electronvolts through the Askaryan effect — the coherent radio pulse produced when a particle cascade develops in a dense dielectric like ice. The Antarctic ice sheet is the detector volume; the balloon is the receiver.
The two anomalous events are designated AAE-061228 and AAE-141220, after their dates, and are also referred to as AE1 and AE2.
AAE-061228. ANITA-I, 28 December 2006, 00:33:20 UTC. Elevation −27.4° ± 0.3°. Earth chord 5,740 ± 60 km. Reconstructed energy 0.6 ± 0.4 EeV. Surface intercept at 82.66° S, 17.28° E, where the ice is 3.53 km deep.
AAE-141220. ANITA-III, 20 December 2014, 08:33:22.5 UTC. Elevation −35.0° ± 0.3°. Earth chord 7,210 ± 55 km. Reconstructed energy 0.56 EeV, +0.30 / −0.20. Surface intercept at 81.40° S, 129.02° E, ice depth 3.26 km.
ANITA assessed the statistical significance of each event against its background as 3.3 and 3.0 standard deviations respectively, with an estimated background of fewer than 0.01 events each.
A separate and weaker class of anomaly appeared in the fourth flight. In December 2016, ANITA-IV recorded four below-horizon events with non-inverted polarity — but only just below, at elevations between −6.1° and −6.7°, where the horizon itself sits at about −6°. The probability of all four being background is 5.3 × 10^-4. Critically, ANITA-IV saw no steeply upcoming events of the 2006 and 2014 kind, and the collaboration has cautioned that the near-horizon events may be a different phenomenon entirely: without a long chord through the Earth, they create no tension with the Standard Model on angular grounds.
Timeline
28 December 2006ANITA-I records AAE-061228 at −27.4° elevation.
20 December 2014ANITA-III records AAE-141220 at −35.0° elevation.
2016ANITA publishes the first flight's anomalous events in Physical Review Letters.
December 2016ANITA-IV flies. It will later be found to contain four near-horizon anomalies and no steep ones.
2018The ANITA-III event is published. The collaboration notes the events imply either a suppressed tau-neutrino cross-section at EeV energies or an extraordinary transient source.
May 2019Shoemaker, Kusenko and colleagues propose a glaciological explanation: sub-surface reflectors and firn density inversions can reflect without inverting polarity.
2019De Vries and Prohira propose coherent transition radiation from air-shower cores intercepting the ice.
January 2020IceCube publishes a seven-year search of its own data in the directions of both events. Nothing. The collaboration states it has ruled out the last remaining Standard Model astrophysical explanation.
April–May 2020The result is picked up by the popular press and, by mid-May, has become a story about NASA finding a parallel universe where time runs backwards. The University of Hawai'i issues a formal correction on 21 May.
February 2021ANITA-IV's four near-horizon anomalies are published.
April 2021ANITA's own HiCal calibration experiment publishes a direct test of the sub-surface-reflector hypothesis and finds it strongly disfavoured.
November 2023Simulations by Ammerman-Yebra, Alvarez-Muñiz and Zas find that transition radiation produces the wrong polarity, disfavouring that explanation.
March 2025The Pierre Auger Collaboration publishes a dedicated search across fifteen years of data and rules out the upward-going-shower explanation for a diffuse flux.
19–20 December 2025PUEO, ANITA's successor, launches from near McMurdo Station.
12 January 2026The PUEO flight terminates after roughly 23 days. The full payload and its data drives are recovered.
July 2026A joint analysis of the ANITA-IV events, the KM3NeT event of February 2023 and the IceCube null result finds a 7.5σ tension with a diffuse Standard Model flux.
The Evidence
The polarity. This is the entire case, and it rests on a single physical fact. Radio emission from a cosmic-ray air shower that reflects off the air–ice interface undergoes a phase reversal. A signal that emerges from the ice going upward does not. Both anomalous events show the non-inverted waveform, in a dataset where roughly fifty comparable cosmic-ray signals show the inverted one. The instrument is measuring the same quantity the same way in every case; two events out of the population disagree.
The angles. −27.4° and −35.0°, each with an uncertainty of three tenths of a degree. These are not grazing, horizon-skimming geometries where reconstruction errors matter. They are steep, and steepness is what converts a curiosity into a physics problem.
The chord lengths. Roughly 5,700 and 7,200 kilometres of rock and iron. Different analyses give slightly different figures — the 2026 joint analysis quotes approximately 5,450 and 7,000 km — but the order is not in dispute. At 0.5 EeV the neutrino mean free path is a few hundred kilometres.
Tau regeneration, and why it does not save the events. A tau neutrino absorbed inside the Earth produces a tau lepton, which decays back into a lower-energy tau neutrino. The flux is therefore not simply attenuated; it cascades downward in energy and can in principle survive long paths. Cline, Gross and Xue examined this quantitatively and concluded that although regeneration does raise survival probabilities at large angles, it "falls far short" of explaining the events within the Standard Model.
The IceCube null result. Regeneration is also what makes the anomaly testable. If ANITA saw one event, IceCube — far larger and running continuously — should have seen many. IceCube searched seven years of data, some 897,000 events over 2,532 days, in the directions of both anomalies. The strongest correlation found was a p-value of 0.08, consistent with background. The flux implied by ANITA's observations overshoots IceCube's upper limit by many orders of magnitude. One caveat is on the record: IceCube was briefly not taking data at the moment of the 2014 event, having begun a run transition about half a second earlier.
The Auger null result. In 2025 the Pierre Auger Observatory reported a dedicated search of fifteen years of fluorescence-detector data for upward-going air showers. Seven and a half million events were reduced to one candidate, against an expected background of 0.27 ± 0.12. Depending on the assumed spectrum and shower-start distribution, Auger should have seen between eight and sixty-nine events if ANITA's signal represented a diffuse flux. Auger's sensitivity exceeds ANITA-III's by factors rising with energy from about two to two thousand.
The Theories
Sub-surface reflection in the Antarctic ice
WeakProposed by Shoemaker, Kusenko, Munneke, Romero-Wolf, Schroeder and Siegert in 2019 and published in Annals of Glaciology. The argument is elegant: reflection does not always invert polarity. Layers within the ice, firn density inversions, ice fabric boundaries or wind ablation crusts could produce a double reflection, or a reflection geometry, that returns a non-inverted pulse. The authors noted the hypothesis was testable by radar survey and warned that future experiments should not treat phase inversion as a sole criterion.
ANITA tested it directly. HiCal is a small balloon-borne transmitter that flies alongside ANITA and fires calibrated radio pulses, producing pairs of directly received and surface-reflected signals. Comparing real reflections against synthetic sub-surface-reflection waveforms, the collaboration reported in 2021 that the data "strongly disfavor" the Shoemaker models: a tilted near-surface ice layer is allowed at the 2–3 per cent level, and the version requiring such features across 7 per cent of the Antarctic continent is clearly disfavoured. Sub-glacial lakes fail twice over, since water has a higher permittivity than ice and would reflect with the wrong sign, and attenuation would suppress the amplitude by five to six orders of magnitude.
Against the rebuttal: it is ANITA testing a rival explanation for ANITA's own anomaly. The Auger collaboration, writing in 2025, lists sub-surface reflection among the proposals without rendering a verdict on it.
Coherent transition radiation from air showers meeting the ice
DebunkedDe Vries and Prohira proposed in 2019 that when a downgoing cosmic-ray air shower intercepts the ice surface, the transverse geomagnetic current crossing the dielectric boundary emits coherent transition radiation, offering what they called a natural Standard Model explanation. In 2023, Ammerman-Yebra, Alvarez-Muñiz and Zas extended a well-established shower-simulation code to handle showers crossing an air–ice interface and found that all such pulses, seen from ANITA's altitude, carry the same polarity as ordinary reflected cosmic-ray signals. The mechanism produces the wrong sign. Auger's 2025 paper records the explanation as ruled out.
Some unidentified radio propagation effect near the ice and the horizon
PlausibleThis is the working assumption of several of the experimentalists involved, and it is honest about being unfinished. Stephanie Wissel of Penn State, an ANITA and PUEO collaborator, put it this way in 2025: "My guess is that some interesting radio propagation effect occurs near ice and also near the horizon that I don't fully understand, but we certainly explored several of those, and we haven't been able to find any of those yet either."
Its strength is that it requires no new physics and accounts for the total absence of corroboration from IceCube and Auger. Its weakness is that it is currently a placeholder rather than a mechanism, and that the steep angles of the 2006 and 2014 events make horizon effects a harder sell than they are for the shallow ANITA-IV set.
Instrumental or systematic error
PlausibleIceCube framed the possibilities as three: an astrophysical source, a systematic error, or physics beyond the Standard Model. Having eliminated the first, it left the other two standing. ANITA's own background estimates for the two steep events are very low, but they are a posteriori estimates, which is a known weakness. The ANITA-IV events carry a non-negligible individual chance of being anthropogenic or of having been misclassified as below the horizon.
Beyond-Standard-Model particle physics
WeakA substantial theoretical literature grew around the anomaly between 2018 and 2023: sterile neutrinos with suppressed active mixing, R-parity-violating supersymmetry producing a long-lived stau, decaying heavy dark matter, inelastic boosted dark matter, leptoquarks, axion quark nuggets, and more.
Most have been tested and found wanting. Cline, Gross and Xue showed that several dark-matter routes underproduce events by an order of magnitude or more, and that dark matter accumulated inside the Earth is excluded by five orders of magnitude in required mass. A 2023 analysis of the ANITA-IV events found the Standard Model-like scenario excluded at about 3σ while a narrow beyond-Standard-Model window survives at 2σ, but the authors state plainly that such an interpretation "relaxes the tension … but does not fully remove it".
The verdict here is weak not because new physics is impossible but because no proposed model both explains the events and survives the null results from IceCube and Auger.
A short-duration transient source
PlausibleIf the neutrino flux were not diffuse but a brief burst, the null results lose much of their force: Auger's exclusion is explicitly scoped to a diffuse flux, and IceCube's searches are constrained by what was overhead when. ANITA raised this possibility in 2018. The difficulty is that it requires two such transients to have occurred, eight years apart, positioned to illuminate a balloon over Antarctica — which is why one 2023 paper called the transient loophole "admittedly baroque". A 2026 joint analysis found that even a population of random rare transients leaves a 5.9σ tension across a hundred thousand simulated realisations.
A parallel universe running backwards in time
DebunkedThis did not originate with the ANITA collaboration and was never a claim about the data. The underlying physics paper — Boyle, Finn and Turok's CPT-symmetric universe, published in Physical Review Letters in 2018 — proposes that the universe before the Big Bang is the CPT mirror of the universe after it. It does not discuss ANITA. A separate preprint connected that framework to the anomaly, and in April 2020 New Scientist covered the IceCube result alongside it. By mid-May the framing had reached the British tabloid press, and because NASA sponsors ANITA, NASA was attached to the story.
Peter Gorham, ANITA's principal investigator, responded through a University of Hawai'i release on 21 May 2020: "This whole parallel universe thing was not invented by us but somehow we have gotten tagged with it. A journalist got it wrong, tied it to us and it has unfortunately snowballed." He added that while he was not opposed to speculation about the anomalies, "our own opinion is that they are more likely to be explained in terms of physics, that is likely to be much less exotic."
A 2026 paper by Massimo Villata revives a CPT-based idea in a different framework, arguing that cosmic rays from CPT-transformed matter in cosmic voids would be observed in reversed time sequence and thus appear upward-going. The author concedes that the claimed directional correspondence with two voids "is not a proof", and calls for PUEO to test it.
What Doesn't Add Up
1. Two events, eight years apart, and nothing in between. ANITA-II saw nothing of this kind, in part because its trigger had been optimised away from air-shower searches. ANITA-IV, the most sensitive flight, saw no steep events at all. Two detections separated by eight years is either a very rare real phenomenon or a coincidence of two different mistakes.
2. The instrument works. Sixty-four reflected cosmic-ray events carry the inverted polarity that theory predicts. The polarity discriminant is validated on the rest of the dataset. Whatever went wrong — if something went wrong — did not go wrong in a way that shows up anywhere else.
3. Two large, independent observatories looked and found nothing. IceCube across seven years and Auger across fifteen. If the events represent a diffuse particle flux, both should have seen it, and the discrepancy is not marginal.
4. Every mundane explanation offered so far has been shown to produce the wrong polarity, the wrong amplitude, or the wrong energy requirement. Transition radiation gives the wrong sign. Sub-glacial water gives the wrong sign. Sub-surface layering survives only at the two-to-three-per-cent level and requires primaries ten times too energetic. The list of what it is not is much longer than the list of what it might be.
5. The ANITA-IV events may not belong to this case at all. They are near-horizon rather than steep, they imply no long Earth chord, and ANITA itself has said they may constitute a new class. Much of the recent literature — including the 2026 joint analysis — treats them separately. Conflating the two sets, which a good deal of public coverage does, makes the anomaly look both bigger and better-studied than it is.
6. The most widely reported fact about the case is wrong. The 2025 press release that generated a fresh wave of coverage states the pulses were detected between 2016 and 2018. Those are the publication years. The detections were in December 2006 and December 2014.
Latest Developments
The most consequential result of the past two years is the Pierre Auger Collaboration's search, published in Physical Review Letters in March 2025. Across fifteen years of fluorescence-detector data it found a single candidate upward-going shower against a background expectation of 0.27, and concluded: "It can thus be argued that the upward-going shower explanation is ruled out for a diffuse flux, unless the distribution of the starting altitude of the showers was shaped such that they would only start several kilometers above ground or the shower profiles had unusual shapes. Both would be inconsistent with showers originating from known particle decays or interactions."
Wissel's reading of that result, given in June 2025, is the clearest statement of where the case stands. The origin of the events, she said, "is still unclear", but Auger's null result "does not indicate that there is new physics, but rather more information to add to the story." And: "It's an interesting problem because we still don't actually have an explanation for what those anomalies are, but what we do know is that they're most likely not representing neutrinos."
In July 2026, a joint analysis by Chattopadhyay, Argüelles and Brdar combined the four ANITA-IV events with the KM3NeT event of February 2023 and IceCube's null result, and found a 7.5σ tension with a diffuse Standard Model power-law flux. Their conclusion is that directional and temporal variation alone cannot reconcile the three datasets. That paper does not address the two steep events at all.
The decisive instrument is now in hand. PUEO — the Payload for Ultrahigh Energy Observations, ANITA's successor, led by Abigail Vieregg of the University of Chicago under NASA's Astrophysics Pioneers Program — launched from the Long Duration Balloon facility near McMurdo Station on 19 or 20 December 2025 (the collaboration and NASA give adjacent dates, most likely a UTC-versus-local artefact). It flew for roughly 23 days at about 120,000 feet and terminated on 12 January 2026, landing some two hundred kilometres from the South Pole. The full payload was recovered, including the data drives.
PUEO carries 108 dual-polarised antennas against ANITA-IV's 48 and a real-time interferometric phased-array trigger. At 30 EeV it is expected to be roughly two orders of magnitude more sensitive than ANITA to a diffuse flux, with pointing accurate to 0.05 degrees. It also carries HiCal-3, a calibration transmitter built specifically to measure Antarctic ice reflectivity — the property on which the glaciological explanation turns.
The PUEO white paper states the priority without ambiguity: "Following up on these apparent upward-going cosmic-ray-like events from ANITA-I and -III is a science priority that motivates the design of PUEO."
As of this writing, no PUEO physics results have been published. NASA has said the analysis may take up to a year.
Sources
- Characteristics of Four Upward-pointing Cosmic-ray-like Events Observed with ANITA — ANITA Collaboration, Physical Review Letters 117, 071101, 2016
- Observation of an Unusual Upward-going Cosmic-ray-like Event in the Third Flight of ANITA — ANITA Collaboration, Physical Review Letters 121, 161102, 2018
- A search for IceCube events in the direction of ANITA neutrino candidates — IceCube Collaboration, ICRC2019 proceedings, 2019
- IceCube rules out last Standard Model explanation of ANITA's anomalous neutrino events — IceCube / University of Wisconsin–Madison, 2020
- Reflections on the Anomalous ANITA Events: The Antarctic Subsurface as a Possible Explanation — Shoemaker et al., Annals of Glaciology, 2019
- Experimental tests of sub-surface reflectors as an explanation for the ANITA anomalous events — ANITA / HiCal, Journal of Cosmology and Astroparticle Physics, 2021
- On the transition radiation interpretation of anomalous ANITA events — Ammerman-Yebra, Alvarez-Muñiz and Zas, 2023
- Can the ANITA anomalous events be due to new physics? — Cline, Gross and Xue, Physical Review D 100, 015031, 2019
- Unusual Near-Horizon Cosmic-ray-like Events Observed by ANITA-IV — ANITA Collaboration, Physical Review Letters 126, 071103, 2021
- Analysis of a Tau Neutrino Origin for the Near-Horizon Air Shower Events Observed by ANITA-IV — ANITA Collaboration, Physical Review D 105, 042001, 2022
- IceCube and the origin of ANITA-IV events — Bertólez-Martínez et al., 2023
- A search for the anomalous events detected by ANITA using the Pierre Auger Observatory — Pierre Auger Collaboration, Physical Review Letters 134, 121003, 2025
- Four, One, and None: Quantifying the Ultra-High-Energy Neutrino Anomaly Across ANITA-IV, KM3NeT, and IceCube — Chattopadhyay, Argüelles and Brdar, 2026
- The Payload for Ultrahigh Energy Observations (PUEO): A White Paper — PUEO Collaboration, Journal of Instrumentation 16, P08035, 2021
- Media incorrectly connects UH research to parallel universe theory — University of Hawai'i System News, 2020
- Strange radio pulses detected coming from ice in Antarctica — Penn State University, 2025
- New Instrument Used Antarctic Ice Sheet to Probe Extreme Universe — NASA Science, 2026
- PUEO — The Payload for Ultrahigh Energy Observations — PUEO Collaboration, 2026
- PUEO's Antarctic flight advances neutrino research — Washington University in St. Louis, 2026
This case is still open.
Want the next file? Comment FILE on the video. The most requested cases become deep dives.
Watch on YouTube