ANITA's impossible upward signals
In 2016 and 2018 the team of ANITA, a NASA balloon that listens for radio pulses from particles hitting the Antarctic ice, reported a few events that looked like particle showers coming up out of the Earth at steep angles where the planet should block them.
2016 CE · -77.86, 167.10 · status: unexplained
The account
The events were recorded on the first (2006–07) and third (2014–15) flights launched from McMurdo. Explanations offered include new particles beyond the Standard Model, unusual reflections in the ice and surface layers, and instrument effects, while press stories spoke of 'a parallel universe'. In 2025 the Pierre Auger Observatory reported a search of its data from 2004 to 2018 that found no matching upward showers, which argues against reading them as real particles emerging from the ground. Their cause is still unresolved.
The sceptical view
Radio reflections from layered ice or snow and unmodelled detector behaviour are considered more likely than new physics, but no explanation has been confirmed.
The researched reading
A real anomaly in a real instrument. Most physicists expect an explanation in how radio waves travel in the ice or in the detector, but a handful of events cannot settle it, and much of the honest answer remains undetermined.
| Explanation | Weight | Why |
|---|---|---|
| Something in the surroundings | 32% | Radio pulses can reflect off layered firn, sub-surface ice structure or surface roughness in ways the original models did not include, making a downward shower look like an upward one. |
| Undetermined | 25% | Only a handful of events exist, and the instrument no longer flies; until a new experiment such as PUEO records similar signals, the cause is genuinely undetermined. |
| A poorly understood natural phenomenon | 20% | Particles beyond the Standard Model (for example a long-lived particle that crosses the Earth) were proposed. The 2025 Pierre Auger search found no matching upward showers, which weakens but does not rule out every version. |
| Something known, misidentified | 15% | Unmodelled detector or reconstruction effects, including errors in the polarity or angle of a faint pulse, could make ordinary cosmic-ray signals look inverted. |
| A known natural phenomenon | 6% | Tau neutrinos emerging at shallow angles are expected physics, but the anomalous events came at steep angles where the Earth should absorb them. |
| Genuinely anomalous | 2% | Press talk of a parallel universe has no support in the data. |
| A hoax or fabrication | 0% | There is no evidence of fabrication; the events were published openly by a large collaboration. |
Official position: The ANITA collaboration lists the events as unexplained; the Pierre Auger Collaboration reported in 2025 that they are unlikely to be upward-going air showers.
Connections in the atlas
Sources
- Wikipedia: Antarctic Impulsive Transient Antenna
- arXiv: Characteristics of four upward-pointing cosmic-ray-like events observed with ANITA (2016)
- arXiv: An unusual upward-going event in the third flight of ANITA (2018)
- Pierre Auger Collaboration: Search for the anomalous events detected by ANITA (PRL 2025)