By no stretch of the imagination could this be understood as a proper and rational basement. The ground under the Bank was a foam of cavities, some recent, some ancient, but mostly ancient. Most were connected to at least one other. This made it possible to navigate through the foam without resorting to shovels and blasting-powder, provided only that one understood the graph of their connexions. This was a job for a banker if ever there was one. Mr. Ham changed direction incessantly, but gave them all some reassurance by only rarely stopping to think, and only once backtracking. Anyone who had even a layman’s knowledge of structures could see at a glance that, over the ages, more than one building had been erected on this site, and many a builder had stayed up late at night worrying about whether the foam could support it. The diverse vaults, arches, timbers, pilings, footings, and rubble-walls that threw back the light of their lanthorns, and forced William Ham into catastrophic reversals, were what those builders had effected when they had gotten worried enough. The timbered tunnels, linteled doorways, and arched passageways through which William conducted them were evidence that other builders had gambled that the inter-locking foundation-works were strong enough, and would not give way if judiciously undermined.