Not sure about the rate per-mile, but literally it's that hot, yes, but not even 15 miles. Just down 5-7km under the oceans, for example at the Mohorovičić discontinuity the temp ranges from 392 to 752F.
Under the ocean is already much, MUCH closer however to the mantle. Oceanic crust is drastically thinner than the land we walk on.
In the Nat Geo article they quote a mine in South Africa reaching up to 55C (131 F) at the bottom and the mine is 4km deep. At a rate 3 degrees Celsius per 100m it should be 120 degrees Celsius over ambient. Which obviously does not add up.
In the Nat Geo article they quote a mine in South Africa reaching up to 55C (131 F) at the bottom and the mine is 4km deep. At a rate 3 degrees Celsius per 100m it should be 120 degrees Celsius over ambient. Which obviously does not add up.