Bordman regarded them for a moment and then inspected the grayish mud underfoot. He lifted his eyes to the inland side of this peculiar stretch of mountainside muddiness. There was a mast on the rock not far away. It held up what looked like a vision-camera. Young Barnes said: "Excuse me, sir. What are those boats doing?" "They're towing an oil-slick out to sea," said Bordman absently, "by towing a floating line of some sort between them. There isn't enough oil to maintain the slick, and it's blown land-ward. So they tow it out to sea again. It holds down the seas. Every time, of course, they lose some of it." "But--" "There are trade winds," said Bordman, not looking to sea-ward at all. "They always blow in the same direction, nearly. They blow three-quarters of the way around the planet, and they build up seas as they blow. Normally, the swells that pound against this cliff, here, will be a hundred feet and more from trough to crest. They'll throw spray ten times that high, of course, and once when I was here before, spray came over the cliff-top. The impacts of the waves are--heavy. In a storm, if you put your ear to the ground on the leeward shore, you can hear the waves smash against these cliffs. It's vibration." Bares looked uneasily at the cliff's edge and the line of boats pushing over an ocean whose waves seemed less than ripples from nearly a mile above them. But the line of boats was incredibly long. It was twenty miles in length at the least. "The slick holds down the waves," Barnes guessed. "It works best in deep water, I believe. The ancients knew it. Oil on the waters." He considered. "Working hard to prevent vibrations! Are they really so dangerous, sir?" Bordman nodded inland. A quarter mile from the edge of the cliff there was a peculiar, broken, riven rampart of soil. It might have been forty feet high, once. Now it was shattered and cracked. It had the look of having been pulled away from where it was withdrawn. There were vertical breaks in its edges and broken-off masses left behind. At one place, a clump of perhaps a quarter-acre had not followed the rest, and trees leaned drunkenly from its top, and at the edge had fallen outward. All along the top of the stone cliff as far as the eye could see there was this singular retreat of soil and vegetation from the cliff's edge. Bordman stooped and picked up a bit of the mud underfoot. He rubbed it between his fingers. It yielded like modelling clay. He dipped a finger into a gray, greasy-seeming puddle. He looked at the thick liquid on his finger and then rubbed it against his other palm. Young Barnes duplicated this last action. "It feels soapy, sir!" he said blankly. "Like wet soap!" "Yes," said Bordman. "That's the first problem here." He turned to a ground-service Survey private, and jerked his head along the coast-line. "How much have other places slipped?"