Lab History: Why Underground?
Most particle physics experiments are looking for extremely small particles that both exist naturally and are produced in high energy physics labs, for experimental purposes, around the globe. The sensitive detectors of the Soudan Underground Physics Lab study these particles. Naturally produced particles from around our galaxy constantly bombard the Earth's surface. These particles, sometimes referred to as cosmic rays, would overload a detector if placed on the surface.
By the time these particles reach the Earth’s surface they have become mostly muons, electrons, positrons, and gamma rays. Most cosmic rays cannot travel very far through rock. If the detectors are located underground, most of the cosmic rays are blocked by the rock.
On the surface, a detector the size of your hand would detect about 2 muons per second. Down in the lab, ½ mile underground, that same detector would detect only 2 muons per week. The image above is a rendering of the underground labs: the angled shaft leads down to the two detector caverns.
Image of Soudan Mine Lab. Mine shaft descends to lab caverns