He let Floyd absorb this unfamiliar view of a familiar object, then continued: "During the past year we have been conducting a magnetic survey of the region, from a low-level satellite. It was completed only last month, and this is the result... the map that started all the trouble."

Another picture flashed on the screen; it looked like a contour map, though it showed magnetic intensity, not heights above sea level. For the most part, the lines were roughly parallel and spaced well apart; but in one corner of the map they became suddenly packed together, to form a series of concentric circles – like a drawing of a knothole in a piece of wood.

Even to an untrained eye, it was obvious that something peculiar had happened to the Moon's magnetic field in this region; and in large letters across the bottom of the map were the words: TYCHO MAGNETIC ANOMALY-ONE (TMA-l). Stamped on the top right was CLASSIFIED.

"At first we thought it might be an outcrop of magnetic rock, but all the geological evidence was against it. And not even a big nickel-iron meteorite could produce a field as intense as this; so we decided to have a look.

"The first party discovered nothing – just the usual level terrain, buried beneath a very thin layer of moon-dust. They sank a drill in the exact center of the magnetic field to get a core sample for study. Twenty feet down, the drill stopped. So the survey party started to dig – not an easy job in spacesuits, as I can assure you.

"What they found brought them back to Base in a hurry. We sent out a bigger team, with better equipment. They excavated for two weeks – with the result you know."

The darkened assembly room became suddenly hushed and expectant as the picture on the screen changed. Though everyone had seen it many times, there was not a person who failed to crane forward as if hoping to find new details. On Earth and Moon, less than a hundred people had so far been allowed to set eyes on this photograph.

It showed a man in a bright red and yellow spacesuit standing at the bottom of an excavation and supporting a surveyor's rod marked off in tenths of a meter. It was obviously a night shot, and might have been taken anywhere on the Moon or Mars. But until now no planet had ever produced a scene like this.

The object before which the spacesuited man was posing was a vertical slab of jet-black material, about ten feet high and five feet wide: it reminded Floyd, somewhat ominously, of a giant tombstone. Perfectly sharp-edged and symmetrical, it was so black it seemed to have swallowed up the light falling upon it; there was no surface detail at all. It was impossible to tell whether it was made of stone or metal or plastic – or some material altogether unknown to man.

"TMA-1," Dr. Michaels declared, almost reverently. "It looks brand new, doesn't it? I can hardly blame those who thought it was just a few years old, and tried to connect it with the third Chinese Expedition, back in '98. But I never believed that – and now we've been able to date it positively, from local geological evidence.

"My colleagues and I, Dr. Floyd, will stake our reputations on this. TMA-l has nothing to do with the Chinese. Indeed, it has nothing to do with the human race – for when it was buried, there were no humans.

"You see, it is approximately three million years old. What you are now looking at is the first evidence of intelligent life beyond the Earth."

MACRO-CRATER PROVINCE: Extends S from near center of visible face of moon, E of Central Crater Province. Densely pocked with impact craters; many large, and including the largest on moon; in N some craters fractured from impact forming Mare Imbrium. Rough surfaces almost everywhere, except for some crater bottoms. Most surfaces in slopes, mostly 10° to 12°; some crater bottoms nearly level.

LANDING AND MOVEMENT: landing generally difficult because of rough, sloping surfaces; less difficult in some level crater bottoms. Movement possible almost everywhere but route selection required; less difficult on some level crater bottoms.

CONSTRUCTION: Generally moderately difficult because of slope, and numerous large blocks in loose material; excavation of lava difficult in some crater bottoms.

TYCHO: Post-Maria crater, 54 miles diameter, rim 7,900 feet above surroundings; bottom 12,000 feet deep; has the most prominent ray system on the moon, some rays extending more than 500 miles.

(Extract from "Engineer Special Study of the Surface of the Moon," Office, Chief of Engineers, Department of the Army. U.S. Geological Survey, Washington, 1961.)

The mobile lab now rolling across the crater plain at fifty miles an hour looked rather like an outsized trailer mounted on eight flex-wheels. But it was very much more than this; it was a self-contained base in which twenty men could live and work for several weeks. Indeed, it was virtually a landgoing spaceship – and in an emergency it could even fly. If it came to a crevasse or canyon which was too large to detour, and too steep to enter, it could hop across the obstacle on its four underjets.

As he peered out of the window, Floyd could see stretching ahead of him a well-defined trail, where dozens of vehicles had left a hard-packed band in the friable surface of the Moon. At regular intervals along the track were tall, slender rods, each carrying a flashing light. No one could possibly get lost on the 200-mile journey from Clavius Base to TMA-1, even though it was still night and the sun would not rise for several hours.

The stars overhead were only a little brighter, or more numerous, than on a clear night from the high plateaus of New Mexico or Colorado. But there were two things in that coal-black sky that destroyed any illusion of Earth.

The first was Earth itself – a blazing beacon hanging above the northern horizon. The light pouring down from that giant half-globe was dozens of times more brilliant than the full moon, and it covered all this land with a cold, blue-green phosphorescence.

The second celestial apparition was a faintt, pearly cone of light slanting up the eastern sky. It became brighter and brighter toward the horizon, hinting of great fires just concealed below the edge of the Moon.

Here was a pale glory that no man had ever seen from Earth, save during the few moments of a total eclipse. It was the corona, harbinger of the lunar dawn, giving notice that before long the sun would smite this sleeping land.

As he sat with Halvorsen and Michaels in the forward observation lounge, immediately beneath the driver's position, Floyd found his thoughts turning again and again to the three-million-year-wide gulf that had just opened up before him. Like all scientifically literate men, he was used to considering far longer periods of time – but they had concerned only the movements of stars and the slow cycles of the inanimate universe. Mind or intelligence had not been involved; those eons were empty of all that touched the emotions.

Three million years! The infinitely crowded panorama of written history, with its empires and its kings, its triumphs and its tragedies, covered barely one thousandth of this appalling span of time. Not only Man himself, but most of the animals now alive on Earth, did not even exist when this black enigma was so carefully buried here, in the most brilliant and most spectacular of all the craters of the Moon.

That it had been buried, and quite deliberately, Dr. Michaels was absolutely sure. "At first," he explained, "I rather hoped it might mark the site of some underground structure, but our latest excavations have eliminated that. It's sitting on a wide platform of the same

black material, with undisturbed rock beneath it. The – creatures – who designed it wanted to make sure it stayed put, barring major moonquakes. They were building for eternity."