Without adequate safeguards, what's down there will come up here (at least to the top of the borehole), and create a new 6km volcano. I'd name it Moria.
If the mantle at the bottom of this new bore-hole is under pressure and liquid, then it will go up the bore hole until the pressure above it equals the pressure below. If the bottom of the bore-hole hits a part of a liquid mantle with a lot of pressure, then we might get a volcano. Hence, Moria.