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Drilling deep

Kola boreholeQ: I recently read that the farthest we have drilled into the earth is 12 miles. Why not further by now?

(December 2012)

Reply by Professor Alastair Fraser, Imperial College London

The maximum depth ever drilled is actually 12,262 metres - just over seven and a half miles - at the Kola Superdeep Borehole in Russia. There are also wells drilled beyond 30,000 feet (five and a half miles) in the Gulf of Mexico. The main constraints on depth are temperature, pressure and the hook load on the rig with all that drill string in the hole. (More on how drilling rigs are constructed here). Ultimately, as the well becomes deeper, the cost becomes more limiting than the technology.

Image: the Kola Superdeep Borehole