I recently mentioned to someone I’ve been about as far underground as the Chilean miners that were recently rescued. Twice I’ve been about half way down Soudan Mine outside Tower Minnesota. The experience is genuinely otherworldly. It was here I experienced absolute darkness. Soudan, once the deepest deep shaft mines in the world (iron mine) is now a state park and is also a physics lab for many interesting physics experiments that benefit from significant shielding from the sun’s rays; such as anti-matter experiments.
More information about this mine turned state park can be found here: http://www.dnr.state.mn.us/state_parks/soudan_underground_mine/index.html