Isobel told the story about Laurence running off to see the rocket when he was a kid. “I kind of kept an eye on Laurence, and when he got done at MIT I offered him my spare room for a while. Actually, Laurence is hardly ever home; this is the first I’ve seen him in weeks. Which can only mean one thing: Red Dwarf marathon.”

Laurence made a big show of rolling his eyes, even though he was kind of in the mood for the long-threatened marathon.

For her part, Isobel had just come back from Greenland, where Milton Dirth was building a vault that was supposed to last ten thousand years, and would only be opened by solving a math problem. “It looks like a bomb shelter, crossed with a Caddy store and a high-end funeral parlor. Everything is shiny steel and chrome, and marble, with glass partitions.”

“What’s in the vault?” Patricia asked. “Seeds? Genetic material?”

“Nope,” said Isobel. “Milton figures whoever opens it in five thousand or ten thousand years will have plenty of edible crops, or they won’t be around at all. It’s all technological and scientific knowledge. Schematics, plans—basically, an instruction manual for re-creating our level of technology, including some ideas for what to do if there are no fossil fuels and certain other elements are unavailable. He’s assuming a roughly early-nineteenth-century science level in whoever finds it. Which might be a stretch, yes. At least the vault will be easy to find: The one piece of electronics in the whole place will be a vertical beam of light, like a searchlight, going off twice a day for at least ten thousand years. That was one of the hardest parts to create.”

“It’s not a serious project,” Laurence said as they crossed Castro Street. “Milton doesn’t think the human race will still be here in a hundred years, much less a few thousand. This is just his way of hedging his bets. Or assuaging his conscience.”

“It’s gotten me three free trips to Greenland,” Isobel said. “Honestly, I think Milton’s opinions depend on how many interns he’s killed today.” She half-winked, to indicate this was a joke and Milton killed no interns.