Tests are typically conducted in vertical shafts, according to the Preparatory Commission for the Comprehensive Nuclear-Test-Ban Treaty (CTBTO). Holes are cut 1 to 3 meters wide and up to a kilometer deep. The atomic devices are assembled on site and placed in the hole, usually accompanied by lead-protected diagnostic canister that contains sensors to record the explosion. The tunnel is then filled with layers of pea gravel, sand and other materials to prevent radioactive material from being released into the atmosphere.

During a test, the explosion energy is released in less than a millionth of a second, according to CTBTO. The temperature will reach about a million degrees within a few microseconds, and shockwaves from the blast, depending on the size, can be detected by seismographs around the planet.

(From time.com)