It depends what you're mining, is the obvious answer. The example in the article was a cement factory - cement is basically made from limestone, and you can fi

It depends what you're mining, is the obvious answer. The example in the article was a cement factory - cement is basically made from limestone, and you can find mountains of the stuff. In general, you're probably looking at sedimentary deposits that have been uplifted - coal was another example someone else gave, you can probably also find mountaintop salt mines.