http://b4hntuy3fimfh2227vf4f74emnya7p35i5brtqujs6leqvtclfwvjbqd.onion/doc/physics/diamonds-crookes/The%20Project%20Gutenberg%20eBook%20of%20Diamonds,%20by%20William%20Crookes..html
The country is almost treeless, and [18] the bare veldt stretches its level length,
relieved only by distant hills on the horizon. The Pipes or Craters The five diamond mines or craters are all
contained in a circle 3½ miles in diameter.
They are irregularly shaped round or oval
pipes, extending vertically downwards to
an unknown depth, retaining about the
same diameter throughout ( Fig. 3 ).