http://b4hntuy3fimfh2227vf4f74emnya7p35i5brtqujs6leqvtclfwvjbqd.onion/doc/physics/diamonds-crookes/The%20Project%20Gutenberg%20eBook%20of%20Diamonds,%20by%20William%20Crookes..html
It may help to realise the enormous
value of the Kimberley Mine if I say that
two claims, measuring together 62 by 31
feet and worked to a depth of 150 feet,
yielded 28,000 carats of diamonds. The roadways across the mine soon,
however, became unsafe. Claims were sunk
100 or 200 feet each side of a roadway,
and the temptation to undermine roadways
was not always resisted.