The deepest holes of all are made for oil and they go down to as much as 25,000 feet. But we do not need to send men down to get the oil out, as we must with other mineral（矿物） deposits（存款）. The holes are only borings, less than a foot in diameter（直径）. My particular experience is largely（主要地） in oil, and the search for oil has done more to improve deep drilling than any other mining（采矿业） activity. When it has been decided where we are going to drill, we put up at the surface（表面） an oil derrick（起重机）.