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” If a hole is deep enough — and a half-kilometer deep hole qualifies — the earth will warm the air inside it. The deeper the hole, the warmer the air. Warm air rises, and cool air sinks, so with a big temperature difference between in-hole air and aboveground air, you get quite a bit of air movement.
Thus, two things are happening. First, the warm air rising from the hole is less dense and gives less lift to helicopter rotors than the cooler air it had been flying through. Since the temperature change is extremely abrupt as the helicopter flies over the hole, the pilot may lose a bunch of altitude before managing to adjust the speed enough (read: increase the spin rate of the rotors) to compensate for the loss of lift.
At the same time, the cool air pouring into that hole from all sides is going to create quite a wind shear. If a helicopter loses enough lift to hit the stream of cold air, it could easily be slammed into the side of the borehole before it ever developed enough lift or power to recover.”