From gold rush to cosmic questions: a South Dakota mine and the search for the secrets of the universe

More than a century after miners flocked to Lead, South Dakota, for gold, the Black Hills town is again a destination for those seeking a different kind of wealth: answers to the universe's deepest mysteries.

On Monday, engineers will begin assembling four giant detectors inside caverns blasted from rock a mile underground, launching the construction of the Deep Underground Neutrino Experiment (Dune).

The project, housed in the former Homestake mine, will study ghostly particles called neutrinos, which are produced in nuclear reactions like those powering the sun and in particle accelerators.

From gold to ghost particles

Neutrinos are the most abundant matter particles in the universe, outnumbered only by photons. They are chargeless and interact so weakly with other matter that about a trillion pass unnoticed through your body each second.

They come in three varieties with minuscule, different masses and change from one variety to another as they travel, a process called oscillation. Dr Kate Shaw, an experimental particle physicist on Dune from the University of Sussex, described this as a very strange behaviour.

A legacy of discovery

Lead has a history with these peculiar particles. In the 1960s, physicist Ray Davis built a detector from a 100,000-gallon tank filled with dry-cleaning fluid, becoming the first to record neutrinos from the sun.

The mine closed in 2001 and was transformed into the Sanford Underground Research Facility (Surf), which is already home to the Lux-Zeplin (LZ) experiment that last month reported the first potential detection of dark matter.

Robin Lucero, Lead’s city administrator, whose family has lived locally for generations, said the town is proud to host such world-class science.

Building a ship underground

Unlike Davis’s apparatus, the Dune experiment is enormous. It will be made from more than 4,000 tonnes of steel and hold a combined 70,000 tonnes of liquid argon chilled to -200C.

Assembling the detectors should take nearly three years, with filling them with liquid argon taking another year. Prof David Newbold, a Dune spokesperson from the UK’s Science and Technology Facilities Council, likened the operation to building a ship underground.

The UK is the largest international partner in the US-led experiment, providing core detector components, critical computing, and the graphite target needed to generate neutrinos at Fermilab in Illinois.

Uncovering the universe's secrets

Once operational, Dune will study an intense beam of neutrinos sent from Fermilab through 800 miles of the Earth’s crust and mantle. Most will shoot straight through, but every so often one will strike an argon atom, creating a flicker of light and a pulse of electric charge.

The project aims to answer fundamental questions, such as which of the neutrinos is lightest and which is heaviest, and to look for subtle differences between neutrinos and their antimatter twins, antineutrinos.

This could reveal fundamental new forces of nature and help solve one of the most enduring puzzles in nature: why everything around us is made from matter and not its “evil twin”, antimatter.

Beyond the beams from Fermilab, Dune will also study neutrinos arriving from space, carrying fresh information on stellar explosions and the formation of black holes.