At the depth of one mile or more, rockbursts occur, since the rock cannot support the immense weight of the rock above it and have a tunnel constructed right beside. It is said that when the rock sounds like its stopped moving around in the walls of the tunnels, that a rock burst is imminent. Fair trade gold, the next market?

http://www.timesonline.co.uk/tol/news/world/africa/article6850786.ece

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