Greed for gold pushes mines deeper...

As this Excite article, points out S. Africa Gold Miners Plunge Ever Deeper, some of the foreign countries who have gold mine interests in South Africa are cautious of the dangers of deep drilling but that can't be said for all:

Some foreign companies have been deterred by the risks here. But Gold Fields, the country's second-biggest producer after AngloGold Ashanti, is ready to set a new record, digging more than 2.5 miles at its Driefontein mine.

A worker was killed there earlier this month by a tremor at just under 2 miles. For comparison, the deepest mine elsewhere in the world is in Ontario, at 1.5 miles.

If you want even more technical information, this post will interest you...