The underground world

The underground world in Foucault’s Pendulum is at once a physical realm of tunnels and excavations and an imagined hidden order beneath ordinary history. It becomes a model for conspiracy thinking: whatever happens aboveground is said to have been arranged below, and unrelated traces are linked until they seem to reveal a directing intelligence. The idea’s force lies in this double meaning. The earth’s actual depths supply images and locations; the conspiratorial imagination converts them into proof of a plan that the characters themselves have made.

From hidden forces to hidden plots

The underground first appears as a theory of causation. Salon, drawing on Athanasius Kircher’s account of nature, distinguishes the Jesuit’s study of subterranean heat and steam from the conspiratorial inference that the world of human affairs is also “planned below.” In this enlarged picture, the underworld is not just a geological region: it is where the causes of visible events are supposed to be concealed. Salon claims that the last Templars took refuge in Agarttha, then revises the location to tunnels that might run beneath Milan. The city’s metro becomes a suspiciously useful question: who chose its route, and who supervised the excavations? Ordinary engineering is reimagined as the surface sign of secret direction.

Salon’s theory draws on the antisemitic Protocols of the Elders of Zion, which he presents as evidence that Jews plot to control governments through underground passages and the destruction of cities. Casaubon recognizes the prejudice behind this account rather than accepting it as neutral historical information. Salon nevertheless treats the underground as self-evident terrain for conspiracy: a plotter must act out of sight, and dominion over the world must mean dominion over what lies beneath it, including subterranean currents. The episode shows how the image of hidden physical space can lend an appearance of coherence to political scapegoating.

The underground also offers a ready-made chain of associations. Salon links tunnels and sacred wells to Celtic sanctuaries, radioactive strata, Glastonbury, Avalon, the Grail, and Jewish invention. He invokes VITRIOL—the alchemical injunction to enter the earth in search of the hidden stone—and folds it into the same system of concealed origins. The connections do not establish a historical continuity; they demonstrate how a charged symbol can make distinct traditions appear to converge. In the characters’ developing method, the subterranean becomes a universal explanation precisely because it can absorb so many different meanings.

A landscape that invites interpretation

The idea gains historical and literary extensions when Casaubon and Belbo search for a modern network below the cities. Casaubon imagines that a group descended from Baconian investigators might have sought the telluric currents without a map or pendulum, by attending to the earth’s own “respiration.” The growth of underground systems in Europe’s great cities then appears to coincide with the scientific age associated with Foucault. Belbo joins in by reading nineteenth-century fiction as an occult disclosure: sewers, tunnels, journeys toward the earth’s center, and hidden realms in the novels of Jules Verne seem to trace the coils of a Serpent beneath the familiar world.

The leaps are deliberately extravagant. A character’s name in The Black Indies is made to resemble “Grail”; the initials of Robur the Conqueror are read as Rosy Cross; and the title’s reversed letters are recruited as another sign. Their imaginative sequence illustrates how resemblances, once granted the status of clues, can pull literature into the same underground map as engineering and esoteric tradition. Casaubon later admits that such links were shortcuts and unwarranted associations. Yet repeated practice has trained the investigators to connect everything with everything else, until the distinction between pretending to believe and believing becomes difficult to maintain.

The theory then turns from excavations to structures reaching upward. If ancient peoples and the Templars dug wells and tunnels, Casaubon reasons, they also established stations aimed toward the heavens, connecting megaliths and capturing stellar influences. Belbo’s sleepless discovery of the Eiffel Tower as a supposed celestial probe extends this conceit: the metal structure becomes an antenna in a planetary apparatus of towers, monuments, and sacred sites. The underground network and the skyward antennae complete one another in the imagined system, joining the earth’s depths to a hidden cosmic communications grid. Their ingenuity is also the warning: a convincing pattern can be manufactured from monuments that have no demonstrated relation to one another.

The underground as a self-sealing explanation

The collaborators’ own account of the Plan exposes the procedure behind this imagined geography. Its first rule connects concepts by analogy; its second accepts a chain if it eventually returns to its starting point; its third requires that the connections have been made before, so repetition can make them seem obvious. Their example runs from a potato through a sequence of resemblances and associations until it comes back to the potato. Under such rules, the underground can connect tunnels, political plots, ancient sanctuaries, popular fiction, and modern infrastructure without requiring independent proof that the links are real.

This method begins as a game and changes through repetition. Casaubon says the three men initially recognized their associations as burlesque, but their long intervals of collecting material and their regular meetings accustomed them to making connections automatically. He describes himself as addicted, Diotallevi as corrupted, and Belbo as converted, while all three lose some ability to tell metaphorical likeness from literal identity. The underground world is therefore not simply one of their theories; it is an example of how a theory’s habits can alter the theorists. The more completely a hidden network explains apparent contradictions, the less room remains for an observation that might disprove it.

That process has a social consequence. Casaubon concludes that they invented a nonexistent Plan, yet other people believed it real and identified fragments of their own confused mythology as parts of it. Once people act on the supposed design, the invented Plan acquires real effects. The underground conspiracy thus develops from an interpretation of concealed forces into a mechanism that can recruit believers and prompt action, even without the secret historical organization it claims to uncover.

What the hidden world conceals

The novel’s most consequential reversal is that the underground’s apparent depth does not guarantee a substantive secret. Casaubon finally accepts Lia’s explanation that the Provins message is only a laundry list: there were no Templar meetings at the Grange-aux-Dîmes, and neither a Plan nor a message existed. This recognition recasts the preceding map-making. The decisive task had never been to discover the Templars’ secret, but to construct one from material that could be arranged to fit.

The conclusion clarifies why the underground remains so compelling. A plot promises an explanation that might dispel frustration or offer salvation; if its secret were known, its seekers imagine, knowledge itself would save them. But a disclosed secret can disappoint, while an undisclosed one can be enlarged indefinitely. Casaubon calls this an empty secret: meaning slips from one association to the next, and initiation becomes the practice of never stopping. The underground’s tunnels embody that open-ended search. They seem to lead toward a hidden center, yet the search can always continue farther below.

The same empty secret explains the power of Belbo’s refusal to reveal what the conspirators believe he possesses. The less he says, the more they magnify the supposed secret; his denial becomes evidence to them that it must be genuine. Aglie’s authority, in turn, depends on the mystery remaining deferred. In this final logic, the underground is less a place where a definite truth is stored than a structure for keeping belief alive by withholding its object. The novel’s caution is not that every search for hidden history is foolish, but that an interpretation capable of turning every clue, denial, and absence into confirmation has escaped the discipline of evidence.