“One of the domes, that’s all,” Cameron said, settling back. But DuBrose didn’t stop staring at the silvery, tattered thing on the hillside.

It was a hemisphere, a hundred feet in diameter, and there were seventy-four of them scattered over America, all exactly alike. DuBrose could not remember when they had been perfectly opaque, mirror silvery shells ; he had been eight years old when they had appeared out of nowhere, all at once, cryptic with their secret that had never been solved. No one had been able to get into them, and nothing tangible had ever come out. Seventy-four shining hemispheres had come from somewhere, causing a near-panic. Another secret weapon of the enemy...

Then the unbroken smoothness of the domes began to be marred. Striae made networks across the polished substance that wasn't matter. And the webs broadened, as though quicksilver were flaking from the back of a mirror, until the shells were tattered and split. It was impossible to see inside them, but there was nothing inside — simply bare ground.

Nevertheless no one had been able to get into a dome. The force, whatever it was, remained constant; something like solid energy made an impassable barrier to solids.