Clavius, 150 miles in diameter, is the second largest crater on the visible face of the Moon, and lies in the center of the Southern Highlands. It is very old; ages of vulcanism and bombardment from space have scarred its walls and pockmarked its floor. But since the last era of crater formation, when the debris from the asteroid belt was still battering the inner planets, it had known peace for half a billion years.

Now there were new, strange stirrings on and below its surface, for here Man was establishing his first permanent bridgehead on the Moon. Clavius Base could, in an emergency, be entirely self-supporting. All the necessities of life were produced from the local rocks, – after they had been crushed, heated, and chemically processed. Hydrogen, oxygen; carbon, nitrogen, phosphorus – all these, and most of the other elements, could be found inside the Moon, if one knew where to look for them. The Base was a closed system, like a tiny working model of Earth itself, recycling all the chemicals of life. The atmosphere was purified in a vast "hothouse" – a large, circular room buried just below the lunar surface. Under blazing lamps by night, and filtered sunlight by day, acres of stubby green plants grew in a warm, moist atmosphere. They were special mutations, designed for the express purpose of replenishing the air with oxygen, and providing food as a by-product. More food was produced by chemical processing systems and algae culture. Although the green scum circulating through yards of transparent plastic tubes would scarcely have appealed to a gourmet, the biochemists could convert it into chops and steaks only an expert could distinguish from the real thing.

The eleven hundred men and six hundred women who made up the personnel of the Base were all highly trained scientists or technicians, carefully selected before they had left Earth. Though lunar living was now virtually free from the hardships, disadvantages, and occasional dangers of the early days, it was still psychologically demanding, and not recommended for anyone suffering from claustrophobia. Since it was expensive and time-consuming to cut a large underground base out of solid rock or compacted lava, the standard one-man "living module" was a room only about six feet wide, ten feet long, and eight feet high.

Each room was attractively furnished and looked very much like a good motel suite, with convertible sofa, TV, small hi-fi set, and vision-phone. Moreover, by a simple trick of interior decoration, the one unbroken wall could be converted by the flip of a switch into a convincing terrestrial landscape. There was a choice of eight views. This touch of luxury was typical of the Base, though it was sometimes hard to explain its necessity to the folk back on Earth. Every man and woman in Clavius had cost a hundred thousand dollars in training and transport and housing; it was worth a little extra to maintain their peace of mind. This was not art for art's sake, but art for the sake of sanity.

One of the attractions of life in the base – and on the Moon as a whole – was undoubtedly the low gravity, which produced a sense of general well-being. However, this had its dangers, and it was several weeks before an emigrant from Earth could adapt to it. On the Moon, the human body had to learn a whole new set of reflexes. It had, for the first time, to distinguish between mass and weight.

A man who weighed one hundred eighty pounds on Earth might be delighted to discover that he weighed only thirty pounds on the Moon. As long as he moved in a straight line at a uniform speed, he felt a wonderful sense of buoyancy. But as soon as he attempted to change course, to turn corners, or to stop suddenly – then he would find that his full one hundred eighty pounds of mass, or inertia, was still there. For that was fixed and unalterable – the same on Earth, Moon, Sun, or in free space. Before one could be properly adapted to lunar living, therefore, it was essential to learn that all objects were now six times as sluggish as their mere weight would suggest. It was a lesson usually driven home by numerous collisions and hard knocks, and old lunar hands kept their distance from newcomers until they were acclimatized.

With its complex of workshops, offices, storerooms, computer center, generators, garage, kitchen, laboratories, and food-processing plant, Clavius Base was a miniature world in itself. And, ironically, many of the skills that had been used to build this underground empire had been developed during the half century of the Cold War.

Any man who had ever worked in a hardened missile site would have felt at home in Clavius. Here on the Moon were the same arts and hardware of underground living, and of protection against a hostile environment; but here they had been turned to the purposes of peaee.

After ten thousand years, man had at last found something as exciting as war. Unfortunately, not all nations had yet realized that fact.

The mountains that had been so prominent just before landing had mysteriously disappeared, hidden from sight below the steeply curving lunar horizon. Around the spacecraft was a flat, gray plain; brilliantly lit by the slanting earthlight. Although the sky was, of course, completely black, only the brighter stars and planets could be seen, unless the eyes were shaded from the surface glare.

Several very odd vehicles were rolling up to the Aries-lB spaceship – cranes, hoists, servicing trucks – some automatic, some operated by a driver in a small pressure cabin. Most of them moved on balloon tires, for this smooth, level plain posed no transportation difficulties; but one tanker rolled on the peculiar flex-wheels which had proved one of the best all-purpose ways of getting around on the Moon. A series of flat plates arranged in a circle, each plate independently mounted and sprung, the flex-wheel had many of the advantages of the caterpillar track from which it had evolved. It would adapt its shape and diameter to the terrain over which it was moving, and, unlike a caterpillar track, would continue to function even if a few sections were missing.

A small bus with an extension tube like a stubby elephant trunk was now nuzzling affectionately up against the spacecraft. A few seconds later, there were bangings and bumpings from outside, followed by the sound of hissing air as connections were made and pressure was equalized. The inner door of the airlock opened, and the welcoming delegation entered.

It was led by Ralph Halvorsen, the Administrator of the Southern Province – which meant not only the Base but also any exploring parties that operated from it.

With him was his Chief Scientist, Dr. Roy Michaels, a grizzled little geophysicist whom Floyd knew from previous visits, and half a dozen senior scientists and executives. They greeted him with respectful relief; from the Administrator downward, it was obvious that they looked forward to a chance of unloading some of their worries.

"Very pleased to have you with us, Dr. Floyd," said Halvorsen. "Did you have a good trip?"

"Excellent," Floyd answered. "It couldn't have been better. The crew looked after me very well." He exchanged the usual small talk that courtesy demanded while the bus rolled away from the spacecraft; by unspoken agreement, no one mentioned the reason for his visit. After traveling a thousand feet from the landing site, the bus came to a large sign which read:

WELCOME TO CLAVIUS BASE

U.S. Astronautical Engineering Corps

1994

It then dived into a cutting which took it quickly below ground level. A massive door opened ahead, then closed behind them. This happened again, and yet a third time. When the last door had closed, there was a great roaring of air, and they were back in atmosphere once more, in the shirt-sleeve environment of the Base.