In what would turn out to be, in retrospect, the northernmost stop on the 16-month Venue itinerary, we drove into the iron ranges and boreal forests of Minnesota to see a 6,000-ton machine buried inside the earth.
The Soudan Underground Mine State Park offers two ticketed tours, each very worthwhile, and we took both of them.
One tour offers a look back at the mine's history, descending 2,300 feet below the surface of the earth to explore the old drifts and stopes. Soudan was Minnesota's oldest and richest mine until U.S. Steel ceased operations in 1963, and the iron extracted here fueled East Coast steel mills, where it was transformed into the nation's railways, machinery, bridges, and weaponry.
The tour begins with a disconcertingly cold, and extraordinarily loud elevator ride shuddering deep into the artificial caverns of this now-derelict site. The ride down is itself spectacular, an all-encompassing roar of noise and darkness, occasionally broken by the film-strip like regular appearance of voids that, we learned, were the entrances of other mine levels we were dropping past. Wondering what was on that level—or that level, or the next level, or this other one—as they flickered by in the gloom allowed the full, nearly overwhelming size of the mine to sink in.
While the historic tour lacks the hokey, interpretive dimension of many other such mine tours, the genuinely hive-like nature of the Soudan Mine—a volumetrically incomprehensible human-carved labyrinth—is only loosely communicated. Only half-joking, we speculated that this might very well be to keep unprepared visitors from experiencing a kind of existential panic upon descent into the 50-plus miles of subterranean chambers.
What sets the Soudan Mine apart, though, is the gigantic high energy physics experiment buried in its bowels. On the accompanying "science tour," visitors have the chance to marvel at the three-story tall, 6,000-ton MINOS "far detector," a kind of catcher's mitt for subatomic particles called neutrinos.
More specifically, these are artificially generated neutrinos fired north from Fermilab outside Chicago. The neutrinos are produced by a complex series of subterranean graphite targets and vacuum pipes just outside Chicago, which transform a spray of protons from Fermilab's "Main Injector" particle accelerator into a focused beam of tiny neutrinos, traveling the 455 miles through the planet between their source and the detector in just 0.0025 seconds.
The neutrinos can make that journey without getting deflected or absorbed by layers of dense bedrock in between because they almost never interact with matter, zipping straight through earth, air, water, and, indeed, people, at a rate of 100 trillion per second, without leaving a single trace.
That same property, however, makes neutrinos extremely difficult to detect—they have been nicknamed the "ghost particle." Not altogether inaccurately compared to a huge camera, the MINOS detector is made from 485 iron plates studded with sensors, each of which is a buffer for slowing down and, in the end, capturing any neutrinos that spiraled through the room. With a trap rate of about one neutrino every two hours, MINOS is able to measure their oscillation speed, which, our guide explained, holds the key to understanding whether these ubiquitous yet elusive particles have mass, and, if so, how much.
While an advanced degree in physics would probably be necessary to tease out the specifics of the experiment and its findings thus far, it's equally awe-inspiring just to gaze on the dense nest of magnetized steel plates, bunched cables, and a multilevel maze of walkways that we were unable to explore, all constructed to capture evidence of an unlikely and otherwise invisible interaction. It's like sci-fi spy technology, with hidden machines picking up and decoding secret broadcasts within the earth.
Elsewhere in the cavern lay the remains of an abandoned earlier experiment designed to witness proton decay (an event that has still not been observed) and a cryogenic dark matter detector, hunting for WIMPs — the heavy, slow, and potentially even more difficult-to-detect cousins of neutrinos.
Interestingly, MINOS, while being an acronym for Main Injector Neutrino Oscillation Search, also refers to Minos, the mythological king who commissioned Daedalus's labyrinth but went on to be a judge in Hades, the underworld of lost souls.
In the end, it was hard not to wonder what will happen to the machine itself—so heavy it seems effectively pointless for anyone ever to dismantle it—and the brightly lit room it is now housed in. Within even 100, let alone 1,000, 10,000, or even hundreds of thousands of years, this huge gate of iron like a camera lens buried inside the earth, will inevitably fall into disuse, its experimental value gone, its costs too expensive to meet.
Then, someday, if it is not removed piece by piece in a mirror image of the construction process that brought it here, it will outlast even the pyramids, just as mysterious to future generations and just as geometrically abstract as those monumental constructions in the sand.