Russia has recently revealed that it’s been sitting on top of “trillions of carats” of diamonds, a secret it has kept until now in order to avoid competing with its other giant diamond deposit, Mirny Mine. The idea of Russia blithely announcing that it has enough diamonds to supply the world market for 3,000 years is rather startling, but even so, that’s not what really grabbed me in the Wired article on the topic. What grabbed me was this:
The [Mirny] mine, now closed due to falling yields, is currently the second-largest excavated hole in the world, and helicopters are forbidden from flying over it in case downward air flow sucks them in.
Seriously??
A search of articles on the topic backs up this assertion. Atlas Obscura has an entry on the mine, accompanied by a photograph that must have been taken through the window of a (very carefully flown) helicopter.
It states that the mine is 1,722 feet (525 meters) deep, and 3,900 feet (1.25 kilometers) across. Then at the bottom of the entry, it says:
Airspace above the mine is off-limits to helicopters, after “a few accidents when they were ‘sucked in’ by downward air flow…”
Ye gods. But what are the physics involved?
After a bit more searching, I found the answer. 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.
So being “sucked in” is a bit of a misnomer. It’s more like it falls in. Either way, that’s freaking scary.
Oregon Expat 
Good heavens! That’s fascinating.
How cool!!!
And cold? In view of the physics involved, which you brilliantly explained, I guess that the phenomena (and the difficulties for the pilot) are enhanced in the particular location of this deep hole:
Mirny, Eastern Siberia: Winter temperatures in Mirny, a town just below the Arctic Circle, average –40 Celsius.
They should install wind turbines around the perimeter of the hole and take advantage of it! New form of renewable energy – hole in the ground energy!
They better not fly over the Grand Canyon then !
Oddly enough, there are two separate companies offering helicopter tours that fly over the Grand Canyon. The only restrictions on where they’re allowed to fly are related to noise levels.
Right. Because the largest canyon in North American does not bear the slightest resemblance in terms of local airflow conditions to a borehole in Siberia.
Warm air usually lifts aircraft. Glider pilots look for regions of warm, rising air to allow their planes to stay in the air longer.
Good explanation of the cause of the danger, however from my experience as a Huey (UH-1) and Cayuse (OH-6A )mechanic during the Vietnam war, my knowledge of their operation begs me to point out that, in flight, rotor speed is constant and is NOT increased to gain additional speed or to increase altitude. Forward speed, hovering, sideways or reverse motion is achieved solely through the cyclic, which controls the tilt of the main rotor head (ie: more forward tilt equals faster forward speed), while altitude is changed through the collective which determines the pitch of the blades providing lift (ie: increased pitch equals increased altitude). Pitch may vary throughout flight, depending on the altitude desired, however RPMs are not, therefore to get out of the predicament, caused by the hole, little is available other than to max out the forward cyclic and or pull full collective. There is one interesting maneuver to rapidly gain altitude and don’t know whether it would work under those circumstances, a “cyclic pop up” creating a tremendous increase in altitude but sacrificing forward motion. It works as long as one has forward air speed, the more forward speed the greater the “pop up” result. It was used to rapidly escape enemy fire while low leveling, and achieved by moving from forward cyclic, immediately into full reverse cyclic thus converting forward inertia into a rapid altitude increase through the sudden increase of air pressure under the rotor blades, attempting to reverse motion while inertia was trying to maintain it. Envision moving from tree top altitude to skyscraper heights in a couple of seconds! Now I don’t know for certain about ALL helicopters, however I suspect the mechanics are all the same. Engine speed only changes when going from startup to flight idle then into actual flight RPM where it’s controlled by a governer, (if the rotor tips exceed the speed of sound, blade failure would result). Therefore greater diameter of the rotor blades equals slower rotor RPM, for routine flight max RPM is the only option. The only time you hear them rev up is when preparing for take off and going from flight idle into actual flight.