“I think so. The first is to control the number of times the individual unit can replicate. So you take one nanobot and you set the replication counter to ten. It makes ten new bots, but each of those can only make nine. And in turn, each of those can only make eight. When you get to zero, replication ends. In that example, you’d end up with millions of bots before it’s all over. If you were to set that initial parameter at a thousand instead of ten, you’re talking numbers that are hard to even imagine.”

Randi frowned and set her beer down. “You said a couple. What about the other structure?”

“That seems to relate to magnetism.”

“They attract each other?”

He shook his head. “I think they measure the earth’s magnetic field.”

“What for?”

“Probably to locate themselves—like birds.”

She let that process for a moment and didn’t like the conclusion she was coming to. “So, if they know where they are, it’s possible that they could be programmed to work only in a certain geographic area and to go dormant outside that area.”

“Very possible.”

They had known the technology worked—the machines were clearly able to self-replicate using steel, plastic, and concrete as fuel. What Jon hadn’t been sure of was whether it could be weaponized. Weapons had to be deadly, but just as important they had to be controllable. If you had a gun, you had to be able to aim it. And that’s exactly what Maple was talking about.

She wanted to ask more, but was unsure how much to reveal. In the end, though, Maple wasn’t an idiot. With the facts he already had, it seemed reasonable to assume that he’d already considered the scenario she was interested in.

“Then you’re telling me I could make it so these things only worked, say, in China. When they crossed over the border into another country, they’d just stop reproducing.”

“Assuming that the system is foolproof. The problem with replication is that it’s hard not to introduce errors. Mutations is probably a better word. Usually, those mutations are neither here nor there. Sometimes they kill the organism, or in this case break the machine. But every once in a while, they could make the machine better.”

“And by ‘better,’ you mean better at reproduction. There could be a mutation that turns off the control system.”

“Exactly. Theoretically, errors could be introduced that would allow the machines to replicate indefinitely or operate outside the programmed geographic borders. Even worse, though, you could get changes to the type of fuel the bots use. What if they became capable of eating rock? Or water? Or flesh? That’s potentially end-of-the-world stuff.”

“What if that happened? How would we stop them?”

“Radiation. And a lot of it. That’s the only thing I’m aware of that can kill these things.”