The no fly zone thing is mentioned on quite a few web sites associated with the mine because of aircraft getting “sucked in”. Does this really happen, and if so by what mechanism is the suction created?
ETA: Wiki also makes this claim: Mir mine - Wikipedia
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Too steep walls can cause air recirculation. Abbottabad Helicopter crash - rotors couldn't generate lift? - Factual Questions - Straight Dope Message Board
A helicopter needs to have normal air below what is being thrust. If it just moves out of the way and up the sides and back through the rotor, you are pushing against air moving away from you. And you fall.