After fifteen years of steady burrowing the Order’s underground warren, buried deep in the Roman ground, had spread over two levels. It housed three hundred people, almost all of them women and children. It was comfortable, once you got used to the dim light and cramped corridors. Of course the Crypt would always be dependent on the surface world, for an inflow of food and water, an outflow of sewage, and for money and building materials and labor: the complex could never cut adrift of the world, like a ship sailing away into an underground sea. But the Council had done all they could to maintain a wide range of links and relationships with suppliers and customers and allies in the outside world, making their sources as diverse as possible, so they were dependent on no one group or person.

As the depth of the Crypt had increased, incidences of flooding or collapse had been dealt with by brute force, with the application of plenty of Roman concrete and brick. Problems with ventilation and heating had been more insidious. Air shafts had been dug out, to be concealed aboveground as artfully as possible. Great fires were lit at the base of some of these shafts, so that the rising air would draw fresh breezes through the tunnels—a practice adopted from deep mines, many of whose engineers Regina had hired to supervise the extension of the Crypt.

But the air shafts alone weren’t enough. There had been a near disaster when a group of five students had been found unconscious, the air in their room foul, a stagnant puddle at the end of a corridor. It had been fortunate for all concerned, Regina thought, that only one student had died—and that her parents, a stoical equestrian family, had been happy to accept the death of their elder daughter as a price to be paid for the safety of their two younger children, both also with the Order. After that incident an elaborate air-monitoring system had been evolved. In every passageway and room there were candles burning, bits of reed dangled from the walls to show the air currents, and caged birds sang in most of the main chambers and corridors.

And it had been found that the simplest way to adjust the environment was by moving people.