I'm assuming you mean if they were to try to go under the crust. The crust is the part of the earth that we live on. This part is like the skin of an apple, and is only about 20 miles thick.

Currently the deepest mine is only about 2.5 miles deep. Even still, the temperature is very high, around 140 degrees Farenheit. It's a very dangerous place to work, and they need cooling to keep it habitable. So you are correct! It's too hot to go deeper with the technology we have.