The famous 1998 Super-Kamiokande announcement of neutrino oscillation is the moment most often cited as the discovery of the effect. But the pre-1998 atmospheric anomaly — the observation that atmospheric muon neutrinos disappear more than expected during their flight through the Earth — had been building for years across multiple independent experiments. Two of the most important contributors, beyond the pre-cursor Kamiokande and IMB experiments, were Soudan 2 in Minnesota and MACRO at Gran Sasso in Italy.
Both experiments operated through the 1990s. Both observed atmospheric muon-neutrino deficits at levels consistent with oscillation. Neither produced a single-experiment 5σ result. But together with the other pre-1998 measurements, they established that the anomaly was real and pointed toward a specific physical explanation. When Super-K’s high-statistics result arrived in 1998, it confirmed what the broader community had already become convinced of.
This article looks at the two experiments and their contributions to building the case for neutrino oscillation.
Soudan 2: the Minnesota tracking calorimeter
The Soudan 2 experiment was a 770-ton tracking calorimeter operating from 1989 to 2001 in the Soudan iron mine, about 700 meters underground in northern Minnesota. The same mine would later host the MINOS far detector for accelerator-based oscillation studies.
The detector was a hybrid design combining drift tubes and scintillator strips, arranged in modular units. The geometry allowed three-dimensional reconstruction of charged-particle tracks with millimeter-scale resolution, plus calorimetric energy measurement.
For atmospheric neutrino physics, Soudan 2 looked at fully-contained events — neutrino interactions occurring inside the detector volume where all the decay products could be tracked and identified. The fully-contained sample provided clean event-by-event flavor identification (electron neutrino vs muon neutrino) and good energy reconstruction.
Soudan 2 measured the ratio of muon-flavored to electron-flavored events and compared it to predictions. The measured ratio was below the prediction at statistical significance levels around 3σ — interesting but not yet definitive on its own.
When Soudan 2’s data was combined with results from Kamiokande and IMB during the early-to-mid 1990s, the picture became more compelling. Multiple independent experiments using different techniques were seeing consistent muon-neutrino deficits. Statistical fluctuations or single-experiment systematic effects could not explain the multi-experiment pattern.
MACRO: tracking upward muons under Gran Sasso
The Monopole, Astrophysics, and Cosmic Ray Observatory (MACRO) ran at the Gran Sasso underground laboratory from 1989 to 2000. The detector was substantially larger than Soudan 2 — about 5,300 tons total — and used a different detection strategy.
MACRO’s primary atmospheric neutrino signal came from upward-going muons produced by atmospheric muon-neutrino interactions in the rock surrounding the detector. Rather than detecting the original neutrino interaction, MACRO detected the high-energy muon that emerged from the rock and crossed through the detector. The direction (upward) tagged the muon as produced by a neutrino — cosmic-ray muons can only come from above.
The advantage of this approach is that the effective target mass is much larger than the detector itself — essentially the entire rock around and below the detector contributes. The disadvantage is that the original neutrino’s energy can only be inferred from the resulting muon’s energy, with less precision than fully-contained events provide.
MACRO’s results, accumulated over a decade, showed a clear deficit of upward-going muon-neutrino events at zenith angles corresponding to long path lengths through the Earth. The pattern was consistent with neutrino oscillation. The statistical significance for the deficit was a few sigma — meaningful but again below single-experiment discovery threshold.
The MACRO collaboration was also doing other physics during this period: searches for magnetic monopoles (the experiment’s namesake program), various cosmic-ray studies, dark-matter searches, and supernova-neutrino monitoring. The atmospheric neutrino results were one part of a broader research program.
How multiple experiments built the case
By the mid-1990s, four experiments had reported atmospheric neutrino measurements with comparable patterns of muon-neutrino deficit:
Kamiokande in Japan: deficit reported in 1988 and confirmed in subsequent papers.
IMB in Ohio: similar deficit reported around the same period.
Soudan 2 in Minnesota: deficit consistent with the other two in the early 1990s.
MACRO at Gran Sasso: deficit in upward-going muons reported through the 1990s.
The combined picture was clear: atmospheric muon neutrinos were disappearing at rates not predicted by the standard calculations. The four experiments used different detection techniques (water Cherenkov, tracking calorimeter, upward-muon tracking) with different systematic uncertainties. The consistency across these very different experimental approaches argued strongly against the explanation being a single systematic effect.
By 1997, the community had largely converged on the interpretation that neutrino oscillation was the most plausible explanation. The remaining question was whether the effect could be confirmed at a single-experiment level of statistical significance. Super-Kamiokande, with its 50,000-ton fiducial mass, was the experiment positioned to provide that confirmation.
Why Super-K got the credit
The 1998 Super-K announcement, presented at the Neutrino ‘98 conference, reported the oscillation discovery at 5σ — well above the discovery threshold and based on a single experiment’s data. Within the community, this was the moment when the multi-experiment “case for oscillation” became a single-experiment “discovery of oscillation.”
The Nobel Prize for the discovery — awarded in 2015 — went to Takaaki Kajita of Super-K and Art McDonald of SNO. The Soudan 2 and MACRO collaborations were not directly recognized.
The credit distribution reflects a common pattern in physics: when multiple experiments build a case over many years and one experiment provides the definitive single-result confirmation, the latter typically receives the Nobel and the popular credit. The contributions of the broader community show up in physics history rather than in the Nobel citations.
That doesn’t diminish the actual contributions of Soudan 2 and MACRO. Both experiments produced peer-reviewed publications that contributed to the global understanding of atmospheric neutrino oscillation. Both helped convince the community that the anomaly was real and was due to a specific physical effect. Both built the experimental infrastructure and analysis techniques that subsequent experiments built on.
What the experiments ran on after
Soudan 2 was decommissioned in 2001. The Soudan mine continued to host other experiments — most notably MINOS — for another 15 years before the mine facility was closed in 2018.
MACRO completed its physics program in 2000. The Gran Sasso laboratory has since hosted many other experiments in the same underground halls, including Borexino, CUORE, GERDA, and LEGEND.
The personnel and institutional knowledge from Soudan 2 and MACRO moved on to subsequent experiments. Many of the techniques developed for these experiments — tracking calorimetry, upward-muon analysis, multi-modal cosmic-ray studies — informed later generations of neutrino detectors.
A community effort
The pre-1998 atmospheric neutrino story is, in many ways, a model of how science builds a case for a controversial conclusion through multiple independent measurements. No single experiment had the statistical power to make a definitive claim. But the consistency across four very different experimental approaches, using different detectors at different sites with different systematic uncertainties, established that the underlying physical effect was real well before the single-experiment 5σ result arrived.
Soudan 2 and MACRO are not as famous as Super-Kamiokande, but their contributions to the broader story are real and substantive. The case for neutrino oscillation, when it became overwhelming, became overwhelming partly because so many experiments had pointed in the same direction over many years.
For the definitive 1998 confirmation, see Super-Kamiokande 1998. For the accelerator-based confirmation that followed, see K2K and MINOS. For the broader oscillation picture, see How neutrino oscillation works and The atmospheric neutrino flux.
Frequently asked
What were Soudan 2 and MACRO?
Soudan 2 and MACRO were atmospheric neutrino experiments operating in the 1990s. Soudan 2 was a 770-ton tracking calorimeter in the Soudan mine in Minnesota, USA. MACRO (Monopole, Astrophysics, and Cosmic Ray Observatory) was a larger detector at the Gran Sasso laboratory in Italy. Both observed muon-neutrino deficits in atmospheric data that, together with results from Kamiokande and IMB, built the case for neutrino oscillation before Super-Kamiokande's 1998 definitive announcement.
How did they observe atmospheric neutrinos?
By detecting the muons produced when atmospheric muon neutrinos interacted in the rock surrounding the detector or in the detector itself. Soudan 2 used a tracking calorimeter that could fully reconstruct events occurring inside its volume. MACRO used streamer tubes to detect upward-going muons that had been produced by neutrino interactions in the surrounding rock — a 'through-going muon' signature that didn't require the original neutrino interaction to occur inside the detector.
What did they find?
Both experiments observed a deficit of upward-going muon neutrino events compared to the expected rate. The deficits were consistent with neutrino oscillation but the statistical significance individually was below the discovery threshold. Combined with Kamiokande and IMB, the results pointed strongly toward oscillation. Super-Kamiokande's 1998 announcement with its much larger detector turned the multi-experiment hint into a definitive discovery.
Why are they less famous than Super-Kamiokande?
Because Super-K's 1998 paper was decisive — a single experiment with high statistics that established oscillation at 5σ. Soudan 2 and MACRO contributed to the same conclusion across the preceding decade, but their individual results were less statistically powerful. Scientific credit tends to accrue to the experiment that produces the decisive single-experiment result, even when many programs contributed to the broader pre-discovery picture.
What happened to these experiments?
Soudan 2 ran from 1989 to 2001 and was eventually decommissioned. The Soudan mine hosted the subsequent MINOS far detector for several more years. MACRO ran from 1989 to 2000 at Gran Sasso. Both experiments completed their physics programs and are now part of the historical record. Their findings contributed substantively to the broader case for atmospheric neutrino oscillation that Super-K eventually established definitively.
Cite this article 5 formats
APA
Neutrino Times Editorial Team. (2026, January 13). Soudan 2 and MACRO: the experiments that built the case for atmospheric neutrino oscillation. Neutrino Times. https://neutrino-times.com/articles/soudan-2-macro-atmospheric-anomaly/
Chicago
Neutrino Times Editorial Team. "Soudan 2 and MACRO: the experiments that built the case for atmospheric neutrino oscillation." Neutrino Times, January 13, 2026. https://neutrino-times.com/articles/soudan-2-macro-atmospheric-anomaly/.
MLA
Neutrino Times Editorial Team. "Soudan 2 and MACRO: the experiments that built the case for atmospheric neutrino oscillation." Neutrino Times, 13 Jan. 2026, https://neutrino-times.com/articles/soudan-2-macro-atmospheric-anomaly/.
BibTeX
@misc{neutrino-times-soudan-2-macro-atmospheric-anomaly,
author = {Neutrino Times Editorial Team},
title = {Soudan 2 and MACRO: the experiments that built the case for atmospheric neutrino oscillation},
howpublished = {Neutrino Times},
year = {2026},
month = {jan},
url = {https://neutrino-times.com/articles/soudan-2-macro-atmospheric-anomaly/},
note = {Accessed: 2026-01-13}
} RIS
TY - GEN TI - Soudan 2 and MACRO: the experiments that built the case for atmospheric neutrino oscillation AU - Neutrino Times Editorial Team PY - 2026 DA - 2026-01-13 PB - Neutrino Times UR - https://neutrino-times.com/articles/soudan-2-macro-atmospheric-anomaly/ ER -