"A tunnel, with about 6 feet of head room and 1800 feet long, would be dug from the U.S. sector in Berlin to the buried cables some 900 feet within the Soviet-occupied sector. A spacious building, ostensibly a military warehouse, was constructed to serve as an on-site headquarters and cover the entrance to the tunnel. It would also explain the to and fro of our personnel, and mask the movement of engineering equipment. Before the digging began, experiments in tunnel construction were conducted in New Mexico and in Surrey, England. The disposal of 3200 tons of earth was dealt with in daily increments... From the outset American personnel, hidden in the apparent “warehouse” and armed with binoculars, maintained a twenty-four-hour-a-day watch of the area stretching from the warehouse to the tap site into the Soviet sector. As dawn broke one morning, the watcher dropped his binoculars, pushed the panic button, and shouted that a dusting of snow was melting on the warm ground above the tunnel. The melted snow marked the tunnel path from the warehouse to the tap site as precisely as if it had been laid by a surveyor’s transit. The crisis was eased when the first half hour of early-morning sunlight melted all of the light snowfall. A few hours later, Harvey had contrived a temporary solution to the problem: in mid-winter and without any explanation, squads of Quartermaster soldiers stripped every available air conditioner from Army premises throughout Berlin. There is less that need be said about the cesspool inadvertently breached as the tunnel engineers navigated beneath a bombed-out farm, or the laundry that had to be established in the warehouse to cope daily with the inexplicably soiled clothing of those working underground."
from A Look Over My Shoulder by Richard Helms