We Got the Shaft! Soudan Underground Mine

As with many places on this trip, we have driven by the Soudan Underground Mine many times, but have never stopped. Today we spent the day at the Soudan Underground Mine State Park. The mine was opened in 1884 and operated until 1962, when US Steel closed it and donated the mine to the State of Minnesota. The Soudan was called the "Cadillac" of mines. It was a safe mine, because the rock was so tight, no supporting timbers were needed. There was never a cave-in. It was also very dry, so workers were relatively comfortable. When the mine closed they were working level 27, more than 2300 feet below the surface. When the mine was originally opened it was an open pit mine, but they quickly discovered that there was extremely rich ore deeper down, and so they went underground. The ore from the mine was extremely rich and contained an extra oxygen molecule, which helped in the refining process. Mining here was not very efficient however. Everything had go to up (ore) and down (equipment, materials and workers) the 4' x 6' shaft, and could weigh no more than 6 tons. This inefficiency was ultimately the demise of the mine. Improved refining techniques made lower quality ore from open pit mines more economical. Refiners learned they could inject oxygen directly into the furnace rather than use oxygen-rich ore. Open pit mines could extract ore on a scale just not possible from an underground mine. Bummer for Soudan.

The state park offers tours down into the mine. Actually there are two tours, and we took them both. The most popular tour is the Historical Mine Tour where you travel down the hoist to the 27th level, 2340' below the surface, then ride a rail car 3/4 mile north to the area that was being worked when the mine was closed. The original mining equipment is still in place and interpreters explain the techniques. Of course they also turn the lights out so you can experience total and complete darkness. It was a constant 50 degrees in the mine, and there were bats fluttering around and clinging to the walls. One lady in our tour freaked out when the bats flew over. I don't know what she was expecting; there was ample warning before we went down.

The second tour was the High Energy Physics Tour. This part gets a little technical. I didn't understand half of it, but here is the condensed and dumbed-down version; the part I sort of thought I maybe understood a little bit. The Soudan Underground Laboratory, operated by the University of Minnesota, has developed several research projects on Level 27 to study sub-atomic particles. This research would not be possible on the surface because of interference from solar and cosmic radiation. Under a half mile of iron ore and very dense Ely Greenstone, cosmic radiation is essentially eliminated. One project is the Main Injector Neutrino Oscillation Search (MINOS). Here they try to capture and study neutrinos, a very small particle similar to an electron, but with no electrical charge. Neutrinos are extremely hard to detect, because they are so small and seldom react with other matter. While neutrinos occur naturally and are extremely common, the neutrinos being detected here are those being fired from Fermilab in Chicago, which travel 457 miles underground through solid rock. The target detector here consists of 485 steel plates, octagonal in shape, 1" thick and 26 feet tall and 26 feet wide. Each plate is laminated with plastic scintillator strips which create light when a particle passes through. Neutrinos are so hard to detect that while ~1.3 E20 (1,300,000,000,000,000,000,000) neutrinos were fired from Fermilab during the first year, only about 200 were detected at Soudan.

The second laboratory is the Cryogenic Dark Matter Search (CDMA). Here researchers are looking for dark matter, the stuff astronomers think makes up most of the matter in the universe. The matter that we are familiar with: air, water,earth and planets, etc,. are made up of well-known elementary particles: protons, neutrons and electrons. It is known that other particles must exist, because of the evidence of their gravitational effect. One theory is the existence of the Weakly Interacting Massive Particle or WIMP. The WIMP is believed to be related to the photon, but with mass up to 10,000 times greater than a photon. A WIMP would only be detectable when it collides with an atomic nucleus. It is thought that 10 trillion WIMPs pass through a kilogram of mass each second, but only a few per year actually bump into anything. CDMS is a project designed to detect these collisions. CDMS has produced detectors the size and shape of a hockey puck that contain very pure crystals of germanium and silicon. When a collision occurs there is vibration which creates minute warming on the germanium crystal. Sensitive thermometers detect this warming. The sensors operate at -460 degrees F, only 1/10th of a degree above absolute zero, making this the coldest place on earth, colder in fact than outer space. Twelve detectors were installed in 2002 and 18 more were added in 2006, but no WIMPs have been detected yet.. Perhaps if they had a Butt-Ugly Low-Life Yahoo (BULLY), he could find a them a WIMP. And you thought you had to go to college to get this kind of education!

The science is all well and good, but the most interesting part of the labs was how they built them. Each lab is in a cavern 270' long, 50' wide and 40' high. Each cavern was excavated solely for the construction of each respective laboratory. Remember all those 26' wide octagonal steel plates? They had to come down that 4' x 6' shaft! 6 tons at a time. All that excavated material? Up the same shaft, 6 tons at a time. All this during the same period when the state park was bringing tourists down the shaft for mine tours. The plates and all the other parts were brought down at night in pieces no larger that 4 x 6 x 33 feet, and then fabricated 1/2 mile down. In fact, they disassembled a full-sized front-end loader, brought it down the shaft and reassembled it. A year later, they tore it apart and took it out, Kinda like building a ship in a bottle, except you can work inside this bottle.

For me the most interesting part of the whole operation was the engine room and the hoist operation. The same hoist system has been in use since the mine was opened in 1884. An operator in the engine room operates the drum that pays out 3000' of 90-ton test cable to raise and lower the cages down the shaft into the mine. The workers signal the operator with a coded series of electric buzzes to tell him to raise or lower the cars. There are no hoist controls in the cars. The operator can tell approximately where the cars are in the shaft by watching a clock-like dial in the engine room. He can tell within a few inches where they are at each level by watching chalk marks on the 12' drum that holds the cable. Once or twice a year they "recalibrate" the cable and put new chalk marks on the drum. Very simple and low tech, but very safe and effective. I spent several hours watching the hoist operation and talking to the operator. It was a tremendous contrast from the highly technical quantum physics research in the labs.