Mirny Diamond Mine
The Mirny diamond mine, also known as "The Mir", is the second-largest mine in the world. It is located in Siberia and operated from the 1960s until the early 2000s. No outsiders can enter the town without a permit, and Russia forbids helicopters from flying over the mine for fear that the air currents would cause the formation of a vortex. There are a number of unsubstantiated reports that some aircraft have actually been sucked into the vortex, never to be heard from again.