Which was not until after the end of a very satisfying weekend. It was noon Monday before I bothered to listen to the messages that had acc.u.mulated on my answering machine.

They were all from Jack Hawkins, and they all said the same thing. Please call him at a toll-free number, as soon as I could, whatever the hour. Despite this insistence, there was no tone of urgency to his voice. There was instead a matter-of-fact, distant quality to it.

Jack was the last person on Earth I expected to hear from. Sure, we had been roommates in college, and the best of friends besides. But despite our heartfelt intentions we didn't stay in touch after graduation. Our careers took different directions, for one thing. He went out west to get a doctorate in geology, while I stayed in New York and worked for a brokerage house.

We hadn't exchanged greeting cards in years, and I never saw him mentioned in our cla.s.s's column in Columbia College Today. He wouldn't have seen mine there either. Though my career as an information scientist was a successful and rewarding one, it had never produced any achievements I could brag to my fellow alumni about. Developing expert systems for collating patent databases with financial a.n.a.lysts' reports posed fascinating intellectual challenges. Unfortunately, n.o.body but an information scientist would be able to share my fascination with them.

"Six seven four three zero." That was all the greeting I got when I punched in the number Jack had given me.

I gave the receptionist my name and asked for Jack Hawkins. There was a perceptible silence before she replied.

"I'll transfer you."

Another short but annoying silence followed. Then I heard Jack's booming voice. "Hi, Dan. Great to hear your voice again. It's been a while."

"Seven years, by my count," I replied. "Where on Earth are you?"

"Nowhere," Jack replied.

"Nowhere?"

"Nowhere on Earth. Now if you asked 'Where on the Moon are you?' I could give you a sensible answer."

"You're on the Moon?"

"Now you've got it."

"So why are you calling me? It's great to hear from you, but interplanetary calls are expensive."

"I was thinking of a conversation we had back in college that I'd like to take up again."

"About what?"

"I'd rather tell you that in person. Would you be able to come up here for a couple of weeks? We'll call it a consulting a.s.signment."

"Consulting for whom?"

"For my employers. A company called Lunar Labs. I imagine you've heard of them."

I certainly had. My employers followed them closely. Lunar Labs wasn't involved in any of the really exciting activity on the Moon, but they were making a bigger profit than anyone else there. The high-profile companies that did the prospecting and the photonics and the materials science work might grab the headlines, but on the bottom line Lunar Labs was the outfit to watch.

Their specially was infrastructure. While Exxon and Hitachi and BHP did what they did best, it was Lunar Labs that provided them with the tools they needed and the room to wield them. Industrial firms weren't their only customers. Lunar Labs also provided technical support for the scientific work of the International Lunar Survey. It was Lunar Labs that carved out the tunnels of Port Armstrong, and Lunar Labs that ran the transportways linking the Lunar stations. Everyone on the Moon knew that if you ate it,Lunar Labs probably grew it; if you drank it, Lunar Labs probably brought it; and if it made you sick, it was almost certainly Lunar Labs that cured you.

Yes, I could see why the company that had sustained the human presence on the Moon would want a geologist. What I couldn't see was what they would want with an information scientist.

"Why is my conversation suddenly so valuable to Lunar Labs?" I asked.

"Let's say that we've got some specialized information needs, and we'd like you to a.s.sess them and advise us how to fulfill them."

This struck me as highly unlikely. Lunar Labs surely had in place some way of providing technical information to its staff on the Moon. If they didn't have a company library back in Houston, they could have contracted with any of a hundred firms that specialized in this sort of work. Even if they really did need a consultant, there were plenty of experts on technical literature for hire. My expertise was in the management of financial information. What use might there be for that on the Moon?

But I couldn't see any harm in taking up Jack's offer. I was due for a vacation, and Port Armstrong was surely as exotic a place as any I might find on Earth. I wouldn't be out of pocket. The work would be interesting. (On the Moon, any work would have to be). And think of all the frequent flyer miles....

"When do I start?" I asked.

"How soon can you be ready?"

"Would next Monday do? I need to get my desktop sorted out."

"Tomorrow would be a lot better."

We compromised on Thursday.

"There will be a car at your door Thursday at seven. Hey, Dan, it will be great to see you again. And have I got a surprise for you...."

The car was there at ten of seven, and I was ready for it.

I've done a fair bit of traveling-every continent but Antarctica so far, and I mean to get there before too many years go by-but I'd always considered the Moon beyond my reach. Sure, I could have found the cost of a ticket and a week's hotel room. For a tenth the cost I could spend three months getting to know Polynesia pretty thoroughly, or visiting every art museum in western Europe. There wouldn't be much to see on the Moon. Some marvels of engineering, certainly. But the Pyramids are marvels of engineering, too, and they're a lot cheaper to get to. Why travel to another planet to see a warren of underground pa.s.sages, when an hour from home the Metropolitan Transit Authority will let me ride through four hundred kilometers of them for just over two dollars?

But if all it would cost me was ten days of my time, I'd happily go to the Moon. At ten of seven that Thursday I was a happy man.

The trip to the airport and the flight to Florida were comfortable, but they were merely preliminaries.

I expected little comfort during the Moon voyage itself, and little comfort is what I got. Even if it was less luxurious and more claustrophobic than a bus in rush hour, I enjoyed every minute of it. There's nothing like interplanetary travel to make a man feel important. I didn't know why I was going to the Moon, but I convinced myself that it must be for some important purpose indeed.

Jack was waiting for me once I had cleared customs. I almost didn't recognize him. His appearance hadn't changed much since our college days. He was still tall, blond, and ruggedly handsome. (I wondered if he was as attractive to Lunar women as he had been to our female cla.s.smates at Columbia.) But he moved differently. He must have been on the Moon for a good while, long enough at least for his body to get thoroughly used to Lunar gravity.

He greeted me as if it had been only a few days rather than the better part of a decade since we'd seen one another. It seemed wise to follow his lead, so I wasn't effusive in my greeting either. It wasn't until we were sitting at the little table in my hotel room, drinking the whiskey he had thoughtfully brought with him, that we started behaving like old friends who hadn't been in touch for years.

Over the first couple of drinks we caught up with each other's news. Then I asked him flat out why he had sent for me.

"Let me show you something," he said in reply. He reached into his briefcase, took out a manilafolder of plans and photographs, and spread its contents on the bed. Then he motioned to me to look at them.

I did, and then asked, "What's so special about these? Is this a new base your outfit is working on? It doesn't look much different from what I saw of Port Armstrong on the way here."

"No, we're not working on it. It was built a while ago. But you're right about one thing. It does look like Port Armstrong, doesn't it? Actually, it's a few thousand kilometers from here. As you said, it's just another warren of rooms and pa.s.sages carved from moon-rock. A lot like this place, really, except that it's on the Far Side."

"What's it called?"

"We call it 'Metropolis.' We don't know its real name."

"What do you mean, 'Its real name?'" I asked.

"Perhaps I forgot to mention one tiny detail," said Jack. "We didn't build it."

Jet lag on a transatlantic flight is nothing compared to the aftermath of the voyage to the Moon.

Between the rigors of the journey itself, the changes in gravity encountered along the way, and the time difference between the Coordinated Universal Time observed everywhere on the Moon and the Eastern Time I had left back home, my body was thoroughly confused. Its confusion had spread to my brain-Jack's whiskey didn't help-and whatever expertise I possessed would do Lunar labs no good until I had enjoyed a good night's sleep. So we made an early night of it. Jack left me with a briefing book, but enjoined me not even to open the cover until morning.

"Order breakfast from room service when you get up. Then you can read it. I'll meet you here at noon tomorrow, and we'll have lunch together."

I suppose that a better man than I would have read the book immediately. How could sleep compete with news of so momentous a discovery? But I was so tired that I could barely get my teeth brushed and my clothes off before falling into bed. It was a full twelve hours before I began to read the report.

"Metropolis" had been discovered by members of an International Lunar Survey team working on their one-to-one-million map of the lunar surface. Its formal name was ILS-2024-A113, but its nickname was appropriate. Metropolis had obviously been a large and populous settlement. Preliminary estimates were that over a thousand personnel had been stationed there. Almost all of it was underground, of course, and in the three weeks since its discovery little had been learned about its secrets. But a few things were obvious.

Its inhabitants were a lot like humans. The air they breathed was interchangeable with that at Port Armstrong, and their ventilation system was still operational. Their lighting system was designed for wavelengths in the range of human vision. Even their plumbing still worked. Their rooms were the right size and height for humans. The placement of walls and ceilings indicated that their spatial sense was much like that of humans, and their stairways suggested a locomotive anatomy compatible to ours. It was the doors that confirmed the resemblance. They were a bit over two meters high, a bit less than a meter wide-and they had doork.n.o.bs that locked from the inside. From this the Survey learned two things: the aliens had hands with opposable thumbs, and they had the concept of privacy.

Despite an intensive search effort, no other alien settlement had been located. It appeared that anything that humans would be able to learn about the aliens would have to be deduced from that one site. And there weren't many resources available on the Moon that could help with the deducing.

Jack explained this to me over lunch. "The problem is that n.o.body, either here or back on Earth, ever expected to find alien artifacts on the Moon. There's n.o.body here prepared to do any serious research on Metropolis. There isn't a single archaeologist or cultural anthropologist on the Moon. We've got a few biologists and a sociologist or two, but they're here to study the effects of prolonged lunar living on humans."

"There must be hundreds, thousands of experts on Earth. Why not bring some of them up here?"

"Two good reasons. First, logistics. How would we get them here? Where would we put them? But more importantly, the one thing about Metropolis that we agree on is that we don't want word to get out that we found it. At least not until we have got a few facts together to tell people. And some sort ofconsensus on what those facts might mean.

"If we brought up a herd of experts from Earth, somebody would be bound to notice. And n.o.body here believes that the archaeological discovery of the millennium can be kept secret once the scientists hear about it. We need a bit of breathing s.p.a.ce, time to figure out how to handle the discovery, and time to get ready to deal with the people it will bring to the Moon.

"And most of all, we need time to prepare people for the impact of the news. Think of what it will mean to humanity to learn for certain that we're not alone."

"So instead of a herd of experts, you've brought me to the Moon. That's certainly flattering, but I don't know what I'm expected to contribute."

"Dan, that's what Colonel Rubin is going to tell you."

I didn't learn anything about Colonel Rubin from his bookshelves. Maps and charts and architectural drawings were spread across nearly every flat surface in the room, and several wallscreens and desktop monitors displayed VR simulations. If there was a bookshelf in his office, I didn't see it. The Colonel himself looked like someone three weeks behind on his sleep; but he seemed genuinely pleased to see us.

"Jack tells me that you might be able to help us solve the biggest riddle our friends in Metropolis have left behind for us. There's a room there that we'd like you to look at. We're hoping you might be able to learn something from it."

"What kind of a room?"

"We're not sure. But it looks like a library."

And so it did. The room was twenty-four meters long, twenty meters wide, three meters high. Its walls were lined with bookshelves, and most of the chamber was filled with double-faced ceiling-high bookstacks that radiated from the center of the room. It was at the center that I found the only feature that would have seemed out of place in a library on Earth: an empty s.p.a.ce, four meters across, surrounded by a circle of lounge chairs. The arrangement seemed more suitable for casual conversation than for study. But what do I know, I thought, about the social psychology of the builders of Metropolis-or the competence of their architects? I smiled wryly as I recalled some of the misbegotten library designs I had seen back on Earth. And, anyway, a library wasn't a thing of shelves and furniture.

The essence of a library was books, and a well-stocked library contained the essence of an entire civilization.

So far as I could see, this library was no exception. But the civilization whose essence it contained was ours. Every book in the room had been written and printed on Earth. Not one single book, sign, label, or sc.r.a.p of paper offered the slightest clue to the aliens' language or culture. It seemed to be a well-balanced collection. Most of the world's major languages and literatures seem to be well represented. So far as I could tell, the major works of human literature were all there: prose and verse, fact and fiction, science and philosophy. But I couldn't see any logic to it. The shelves were neatly arranged, with the books standing up straight and s.p.a.ced evenly. One third of each shelf was empty, just like the library science textbooks recommend, to allow room for expanding the collection. I couldn't believe that was accidental. It looked as though somebody on the staff read the shelves every day, and put everything back just so.

I could make no sense out of the way the books were arranged on the shelves. There was a copy of Montaigne's Essays next to the I Ching, and a 1958 Houston telephone directory next to War and Peace. Occasionally there seemed to be several related t.i.tles in sequence, but this happened only just often enough to seem random in itself. Perhaps the books were simply shelved in order of acquisition.

But if that were the case, why was there that empty s.p.a.ce on each shelf?

It was a pretty good collection. It would have made a great public library for a small town. Not just any town: a seaport, maybe, or a college town-someplace whose inhabitants spoke an awesome variety of languages. And it would have to be a town whose inhabitants lost all their interest in literature somewhere around 1965.

I wondered how the aliens had managed to put this library together. There must have been twentythousand books. That's a lot of ma.s.s to lift up from Earth. Books are pretty heavy.

And of course there was the obvious question: why did they go to all the trouble? And the obvious answer: for the same reason they went to the trouble of building Metropolis. To study us. The books they had gathered together suggested that they had learned a lot about us.

So now I had to work out an answer to the question that Colonel Rubin had put to me. How might we use this library to find out something about them?

"Jack was telling me about a conversation that you and he once had when you were roommates at Columbia," Colonel Rubin had said during our first meeting.

"You were telling me about a book you had read," Jack said. "A history of libraries. You were explaining the ways that libraries had been used in various times and places. Something about curses and omens, as I recall."

"Oh, yes. The first recorded use of a library. Some king in ancient Babylonia had a collection of curses inscribed on clay tablets. Whenever he wanted to curse one of his enemies, he would have the royal librarian retrieve a tablet with a suitable imprecation. And then there was the other king whose heptomancers would predict the future by slaughtering a sheep and examining the shape and color of its liver. They would compare it with clay models they had made of livers from previously slaughtered sheep.

When they found a model that resembled the liver in question, they'd consult the records to see what had happened after the earlier sheep had been killed. Not the way we do science today, but there was a method to it."

"And there was something about the Imperial Library of China."

"Well, the emperors derived their authority from the Mandate of Heaven, as set forth in the Confucian cla.s.sics. And to ensure that the Cla.s.sics were interpreted so as to justify their claims, they used the Imperial Library to collect and preserve and publish approved texts of the Cla.s.sics. They even had them carved onto stone tablets, so that n.o.body could sneak any unauthorized changes into the text."

"You had a lot of other examples, too. Well, I remembered that conversation, and that got me to wondering what role this library played in the culture of Metropolis. So I suggested to the Colonel here that you might be in a position to shed some light on that. Or at least to offer a few suggestions."

So far, the only light I had shed was the result of twisting a k.n.o.b next to the entrance. If there was any reason behind the arrangement of the books, it totally escaped me. And the reason why they had been left behind was an even greater mystery.

Why had the inhabitants of Metropolis abandoned their library for us to find, rather than taking it with them? It couldn't be a question of too much ma.s.s. If they could bring up all those books from Earth, surely they could lift them off the Moon. The Lunar gravity well is nowhere near as deep as Earth's. So they must have intended for us to come across them. Were they trying to tell us something?

I hoped it wasn't anything about their taste in literature. "If you were on a desert planet, what twenty thousand books would you bring with you?" Certainly literary quality couldn't have been the criterion.

Their selection was just too random, less a choice than a cross-section. Perhaps their purpose was to provide a scientific sample of Earth's published literature. They were using the term rather broadly, I thought, glaring at the Houston telephone directory.

All this was getting me nowhere. But something was gnawing at the back of my mind. After a moment of concentration it came to me.

"You know," I remarked to Jack, "there's a precedent for our situation."

"Indeed? When was the last time somebody had to unravel an alien civilization?"

"About two hundred years ago. There was a French expedition to Egypt, the one that pretty much discovered ancient Egyptian civilization. They found a lot of inscriptions, all in hieroglyphics, which n.o.body could read. At least not until Champollion found a black stone slab-I suppose you could call it a monolith-at a place called Rosetta. It was inscribed with the same text in three languages: hieroglyphics, demotic (that's what the ancient Egyptians spoke every day), and Greek. It took him twenty years, but Champollion managed to decipher the whole thing, and from that discovered the entirehieroglyphic alphabet. After that, archaeologists were able to unravel the history of ancient Egypt.

"That's what we need. A Rosetta Stone."

"Well, we haven't got one," Jack said.

I sought out the lounge chairs at the center of the room, to sit down for a few minutes and get my thoughts in order. I sprawled out comfortably and began to close my eyes, the better to concentrate. I was just on the edge of slumber when I saw the flickering light out of the corner of my eye.

Looking up, I saw the concave hemispherical opening that had formed silently in the ceiling above me. In its center shone a sphere of many colors. Strips of multicolored light radiated from it at all angles, some resembling miniature pennants, others Mobius strips. Sharp stubby spikes bristled from the sphere, and a long double helix trailed almost to the floor. There were one or two extrusions that reminded me of Klein bottles. Each of the streamers bore a pattern that looked like a spectrum, and the sphere itself was a mosaic of colored shapes. I had once seen something similar, at a gallery in Boston. But the object above me was a couple of orders of magnitude more complex.