Phillip was thinking as he drove out at the head of his team to the site the next morning.

He was feeling bleak. His discoveries had not helped him to sleep. He wondered what, if anything, lay within the metal ball which he had always known as the Moon. Now it was something else. It had already lost its original magic when he first set foot on it. For centuries the moon had been a symbol of romance, or horror, the white eye they sailed above the ships, the beautiful orb that reflected in pools. Now it was just another barren, dead lump of rock. It flew round the Earth and it reflected light, and it pulled the tides. Now the flat sided construction had been found, it was less than that. Now it was just a construction, built by some intelligence which Man knew nothing about. He knew he shouldn't say 'just' because the engineering was incredible, but why must everything be reduced to machines and laws? He hated the way logic forced him to follow truth.

The polygon flat seemed to mock him as he pulled up near the centre. Here was insignificant Man, rolling about on the top of a machined wall, without a clue as to its origin or purpose, and he had some crude tools with him to chip, or drill, or burn he knew not what. It didn't throw all the great advances of the human race into a good light, that was for sure.

Ralph was driving a modified machine this time. It had a circular brush on it, which began to spin as soon as he had pulled into position. It came down slowly on its hydraulic arm and began to raise a huge, billowing cloud of dust, which totally obscured the sweeper.

The other machines began to work on electronic scanners and moved out into the floor of the wide crater. They dug and pushed about in the clouds of grey smoke-like dust, following the adjacent polygon until it was completely uncovered. By the end of the eight hours they had it cleared, and the sides of two complete polygons were revealed.

Phillip spent the rest of his time driving about, testing the flat with every device he had on board. He worked on a grid which the computer had constructed for him, moving a short distance between co-ordinates until half the polygon had been inspected. He was near the centre when he detected a small, straight line in the surface.

Reversing, he got out and scraped at the surface with his boot. He wondered if it was a scratch, though the possibility seemed unlikely. It was a groove. It ran straight, and hit a right-angled comer, he felt with his fingers along the second line until it too hit a corner. When he had followed the line further, it turned again, and he was back where he had started. Now he would need