While the February 2008 Now You Know discussed tunnel boring, it seemed appropriate to look at deep drilling in light of the recent Chilean miner rescue. It is especially appropriate because without thermal processing, the rescue would have taken much longer or might not have been possible at all.
The definition of a deep hole is anything more than four to five times its diameter. Needless to say, the more than 2,000-foot hole required to reach the Chilean miners was deep. There are several techniques for drilling deep holes. The choice depends on hole size and economics.