Thud!

have arrived.”
    The elevator stopped with barely a jerk.
    Vimes stared.
    Ankh-Morpork was built on Ankh-Morpork. Everyone knew that. They had been building with stone here ten thousand years ago. As the annual flooding of the Ankh brought more silt, so the city had risen on its wall until attics had become cellars. Even at basement level today, it was always said, a man with a pickaxe and a good sense of direction could cross the city by knocking his way through underground walls, provided he could also breathe mud.
    What had this place been? A palace? The temple of a god who’d subsequently slipped everyone’s memory? It was a big space, dark as soot, but there was a glow that managed to show beautiful vaulting in the roof above. A strange glow.
    “Vurms,” said Ardent. “From the deep caves in the mountains around Llamedos. We brought them with us, and they breed very fast here. They find your silt quite nourishing. I’m sure they shine more, too.”
    The glow moved. It did not illuminate much, but it showed the shape of things, and it was heading toward the elevator, flowing over the wonderful ceiling.
    “They head for heat and movement, even now,” said the hooded dwarf.
    “Er…why?”
    Ardent gave a little laugh.
    “In case you die, Commander. They think you are some rat or small deer that has tumbled into their cave. Nourishment is rare in the Deeps. Every breath you exhale is food. And when eventually you expire, they will…descend. They are very patient. They will leave nothing but bones.”
    “I was not intending to expire here,” said Vimes.
    “Of course not. Follow me, please,” said Ardent, leading them past a big, round door. There were more doors on the other side of the room, and several gaping tunnel mouths.
    “How far down are we?”
    “Not far. About forty feet. We are good at digging.”
    “In this city?” said Vimes. “Why aren’t we trying to breathe underwater? And calling it water is giving it the best of it.”
    “We are very good at keeping water out, too. Alas, it appears we are less good at keeping out Samuel Vimes.” The dwarf stepped into a smaller room, its ceiling thick with brilliant vurms, and motioned to a couple of dwarf-sized chairs. “Do sit down. Can I offer you refreshment?”
    “No, thank you,” said Vimes. He sat down gingerly on a chair that brought his knees up almost to his chin. Ardent sat down behind a small desk made of stone slabs and, to Vimes’s amazement, took off his headgear. He looked quite young, with a beard that was actually trimmed. Angua watched him, breathing slowly.
    “How far do all these tunnels run?” Vimes said.
    “I don’t propose to tell you,” said Ardent levelly.
    “So you’re undermining my city?”
    “Oh, Commander! You’ve been to the caves in Uberwald. You’ve seen how dwarfs can build? We are craftsmen. Do not think that your house is about to collapse.”
    “But you’re not just building basements! You’re mining!” said Vimes.
    “In a sense. We would say we are mining for holes. Space, Commander, that is what we are digging for. Yes, we are mining for holes. Although our bores have found deep treacle, you will be interested to hear—”
    “You can’t do this!”
    “Can we not? But we are doing it nevertheless,” said Ardent calmly.
    “You are burrowing under other people’s property?”
    “Rabbits burrow, Commander. We dig. And, yes, we are. How far down does ownership go, after all? And how far up?”
    Vimes looked at the dwarf. Calm down, he thought. You can’t deal with this. This is too big. It’s something for Vetinari to decide. Stick to what you know. Stick to what you can deal with.
    “I’m investigating reports of a death,” he said.
    “Yes. Grag Hamcrusher. A terrible misfortune,” said Ardent, with a calmness that was enraging.
    “I’ve heard it was a vicious murder.”
    “That would be a fair description.”
    “You admit it?” said Vimes.
    “I’ll choose to assume that by that you mean ‘do I admit there has been a murder,’ Commander. Yes. There has. And we are dealing with it.”
    “How?”
    “We are discussing the appointment of a zadkrdga ,” said
    Ardent, folding his hands. “That is ‘one who smelts.’ One who finds the pure ore of truth in the dross of confusion.”
    “ Discussing? Have you sealed off the scene of the crime yet?”
    “The smelter may order that, Commander, but we already know that the crime was committed by a troll.”
    Ardent’s