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pieces of things that they’re still studying; and some trinkets. That’s all.”
    She said, “That’s the way I heard it. And one more thing. None of the discovery dates on these things is less than fifty years old.”
    She was smart and better informed than I had expected. “And the conclusion,” I agreed, “is that the planet has been mined dry. You’re right, on the evidence. The first diggers found everything there was to be found … so far.”
    “You think there’s more?”
    “I hope there’s more. Look. Item. The tunnels. You see they’re all alike—the blue walls, perfectly smooth; the light coming from them that never varies; the hardness. How do you suppose they were made?”
    “Why, I don’t know—”
    “Neither do I. Or anybody else. But every Heechee tunnel is the same, and if you dig into them from the outside you find the basic substrate rock, then a boundary layer that’s sort of half wall-stuff and half substrate, then the wall. Conclusion: The Heechees didn’t dig the tunnels and then line them, they had something that crawled around underground like an earthworm, leaving these tunnels behind. And one other thing: They overdug. That’s to say they dug tunnels they didn’t need, lots of them, going nowhere, never used for anything. Does that suggest anything to you?”
    “It must have been cheap and easy?” she guessed.
    I nodded. “So it was probably a machine, and there really ought to be at least one of them, somewhere on this planet, to find. Next item. The air: They breathed oxygen like we do, and they must have got it from somewhere. Where?”
    “Why, there’s oxygen in the atmosphere—”
    “Sure. About a half of one percent. And better than 95 percent carbon dioxide; and somehow they managed to get that half of one percent out of the mixture, cheaply and easily—remember those extra tunnels they filled!—along with enough nitrogen or some other inert gas—and they’re present in only trace amounts—to make a breathing mixture. How? Why, I don’t know, but if there’s a machine that did it, I’d like to find that machine. Next item: Aircraft. The Heechee flew around the surface of Venus at will.”
    “So do you, Audee! Aren’t you a pilot?”
    “Sure, but look at what it takes. Surface temperature of two-seventy C. and not enough oxygen to keep a cigarette going. So my airbody has two fuel tanks, one for hydrocarbons, one for oxidants. And—did you ever hear of a fellow named Carnot?”
    “Old-time scientist, was he? The Carnot cycle?”
    “Right again.” That was the third time she’d surprised me, I noted cautiously. “The Carnot efficiency of an engine is expressed by its maximum temperature—the heat of
combustion, let’s say—over the temperature of its exhaust. Well, but the temperature of the exhaust can’t be less than the temperature of what it flows into—otherwise you’re not running an engine, you’re running a refrigerator. And you’ve got that two-seventy ambient air temperature; so you have basically a lousy engine. Any heat engine on Venus is lousy. Did you ever wonder why there are so few airbodies around? I don’t mind; it helps to have something close to a monopoly. But the reason is they’re so damn expensive to run.”
    “And the Heechees did it better?”
    “I think they did.”
    She laughed again, unexpectedly and once more very attractively. “Why, you poor fellow,” she said in good humor, “you’re hooked on the stuff you sell, aren’t you? You think that some day you’re going to find the mother tunnel and pick up all this stuff.”
    Well, I wasn’t too pleased with the way things were going; I’d arranged with Vastra’s third to bring the girl here, away from her boyfriend, so I could pick her brains in private. It hadn’t worked out that way. The way it was working out, she was making me aware of her as a person, which was a bad development in itself, and worse than that, making me take a good look at