(from the article `tunnels and underground excavations`) ...operations, in which shafts 5,000 to 8,000 feet deep are common and are generally 20 to 30 feet in diameter. South African procedure has produced ... Found on http://www.britannica.com/eb/a-z/s/101
(from the article `tunnels and underground excavations`) ...operations, in which shafts 5,000 to 8,000 feet deep are common and are generally 20 to 30 feet in diameter. South African procedure has produced ...