The BBC News article The Deepest Hole we have Ever dug says:
This is the Kola Superdeep Borehole, the deepest manmade hole on Earth and deepest artificial point on Earth. The 40,230ft-deep (12.2km) construction is so deep that locals swear you can hear the screams of souls tortured in hell. It took the Soviets almost 20 years to drill this far, but the drill bit was still only about one-third of the way through the crust to the Earth’s mantle when the project came grinding to a halt in the chaos of post-Soviet Russia.
The first image below shows the top of the Kola Superdeep Borehole, and the caption says that it is welded shut.
Is there any way to form a reasonable hypothesis what would happen if it were opened today? Would the 12 km deep well have filled with water, or helium, or natural gas, or is it likely to be filled with air? Would it be under tremendous pressure on the other side of the bolted-down cap?
above: The borehole still exists - but the entrance has been welded shut (Credit: Rakot13/CC BY-SA 3.0), below: The borehole is located in the wilds of Russia's northern Kola Peninsula (Credit: Getty Images)