[Melatonic]

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.

[m4rtink]

They reached 160C rock when building an industrial railway in Japan:

https://en.m.wikipedia.org/wiki/Kurobe_Seny%C5%8D_Railway

Cooled down to 40C since the 60s.