Scientists Probe Universe 4,850 Feet Underground at America’s Deepest Lab

Q: What bizarre anomalies from a flooded Finnish mine are driving scientists to build new detectors deep underground?

The anomalies were unusual **high neutron multiplicity events** seen in Finland’s **Pyhäsalmi mine**—bursts in which a single cosmic-ray muon appeared to trigger unexpectedly large showers of neutrons underground. Researchers do not yet know whether those events came from an ordinary but poorly understood background source or from something more exotic. That uncertainty is what is driving **NEMESIS** at the Sanford Underground Research Facility in South Dakota. The new experiment is built to measure these muon-induced neutron bursts in detail, because such events can mimic or obscure the signals sought in ultra-sensitive underground experiments, including dark matter searches. At Pyhäsalmi, scientists reported “unusual features” in the neutron data before the mine shut down and later flooded, cutting off further study there. NEMESIS is essentially a continuation of that interrupted work. By placing one detector deep underground at SURF’s **4850 Level** and planning another at the **1700 Level**, researchers can compare neutron activity at different depths and test how the **cosmic-ray muon flux** changes the rate and character of the events. The aim is practical as much as speculative: improve background models for rare-event physics and determine whether the Finnish anomalies were simply an unrecognized neutron background—or, in a much less likely scenario, a hint of new physics such as dark matter.

3 articles · Updated · South Dakota Public Broadcasting · Jul 28

4,850 feet below Lead, South Dakota, scientists are conducting experiments at the Sanford Underground Research Facility, the deepest underground laboratory in the United States.
SURF’s depth helps researchers tackle major questions about the universe by providing a shielded environment for sensitive physics experiments.
The lab’s research portfolio recently expanded with NEMESIS, an experiment studying high neutron-multiplicity events and possible dark-matter-related anomalies alongside work such as LUX-ZEPLIN.