In this post from TechReview - Underground Wi-Fi: Cities may wait, but mines are getting full wireless broadband coverage:
the post discusses how wi-fi is being used underground in mines for the safety of the miners...
"Miners aren't blogging from the tunnels -- yet. In Kiruna, information from drills and trucks -- such as their positions and the weight of their loads -- is relayed via wireless base stations to a computer in a control room above ground. (Weight is an important datum; it tells the operator how good the ore is. The heavier the better.) With Wi-Fi networks, fewer miners have to face the risks of working underground -- and those who do have a more durable link to the outside.
LKAB, the company that operates the Kiruna mine, has experimented with wireless networks before, but Wi-Fi offers cheap standardized components and is the newest tool for boosting mine safety and productivity"