Dugan nodded his agreement. Sarah, who knew anyhow, said nothing.
Swanson pulled out a photostat from the bottom of the pile. It was a pale photograph with the overlay of a map printed by hand in glaring white. The map showed a big underground city which ran underneath two or three peaks in the range, depending on what you counted as peaks.
Swanson explained what he had done. For weeks he had gone over the photographs, finding tell-tale lines of color difference in the trees, odd shadows which added up to the modification of natural terrain. Two photographs together showed shadow lines which hinted at camouflaged excavation, damaged trees showing power lines, a thickened brook hinting at water overflow.
The colors of the photographs ranged from pale greens to weird purples. Swanson explained:
"These aren't meant to pick up the actual colors, but to range from infra-red all the way up through the visible spectrum. We figured that the Russians would build their camouflage doctrine on the assumption of black-and-white photography or color perception by the naked eye. They couldn't fool us on the color pattern and the black-and-white pattern, not at the same time. See how this film shows up the differences in foliage tints?"
Dugan and Sarah nodded.
Swanson ran his finger along the patterns which neither of the others could see till he pointed them out; but it was amazing how clear each pattern remained, once it had been pointed out.
A fantastic city lay beneath the leaves. Swanson's voice rang with technical enthusiasm as he explained the enormous care which had gone into the building of Atomsk. Purely by air view, it would never have been detected. A renegade, a panicky Soviet pilot, and a Chinese coolie had had to show the way; otherwise it never would have been found.
Swanson said, very emphatically, "Do you see — they have hidden it from their own people, too? They have thousands of planes and thousands of pilots in this part of the world. They could not post Atomsk as a prohibited area without a million or so people finding out about it. They had to leave it so that even Russian aircraft would find nothing. The best way to keep a secret is to have no secret to keep, in the first place. No lights. No roads. No warnings. Just the empty forest, and on the ground the secret police shunting people this way and that with a thousand prohibited zones. Any one of them could have been Atomsk. But this one is it."
Sarah said, "If there's any question of needing more information, why don't we fly another plane in?"