Actually Vale Inco (Creighton Mine-Sudbury) is working on a project to mine at 10,000 ft or 3040m below surface!

Vale web-site: http://www.vale.com/vale_us/cgi/cgilua.exe/sys/start.htm?sid=345

"Our Creighton Mine in Sudbury, Ontario is a good example – continuously mined for more than 100 years, Vale Inco is currently mining at depths of 7,600 feet below surface at Creighton and exploring the feasibility of mining at depths to 10,000 feet. Hydro-metallurgy route (lixiviation under pressure – HPAL) in the HPAL process, the mineral can first be enriched through physical benefiting, with the removal of silica (if existent)