A bit of conspiracy theory to open this month of ides and signs and portents: For decades, a giant pendulum slowly swung inside the Smithsonian’s Museum of History and Technology. When the museum was renovated to become the Museum of American History, the pendulum disappeared. Where did it go? And why, in those closing days of the Bush administration, did it go?

It was not just any pendulum. It was a Foucault pendulum, designed by a 19th-century French scientist to show the rotation of the Earth. Now, the physical world, ancient philosophers and modern scientists agree, is governed by chaos; just so, the human world is swept along by mysterious forces, dark eruptions of the spirit, cruel fixations and mad superstitions. So Umberto Eco suggests in Foucault’s Pendulum, a sprawling philosophical novel that appeared a quarter-century ago in its original Italian and went on, like his earlier Name of the Rose, to find readers around the world.

The novel centers on three editors at a Milanese vanity publisher. They pass their quiet days by inventing oxymoronic courses, such as “Urban Planning for Gypsies,” for their projected School of Comparative Irrelevance and by flattering and bilking wishful authors—always a subject with possibilities. Hardened cynics, the narrator Casaubon and his colleagues Belbo and Diotallevi regard their work with arrogant zeal: “Transforming books with a word here, a word there….If you fill the world with children who do not bear your name, no one will know they are yours. Like being God in plain clothes.”

Their playful life is soon interrupted by one Col. Ardenti, whose great dream is to publish a history of the Knights Templars, a military order of monks enriched by the Crusades and crushed by papal order in the early 1300s. Certain that the Knights Templars still exist and secretly rule the world—a story that has been taken and run with by lesser authors since—Ardenti urges his manuscript on the doubtful editors. Soon thereafter, the three are drawn into a weird whirlwind that is at least partly of their own making, and it is up to Casaubon to save them all.

Guided by hints Belbo has hidden in his computer, called Abulafia after the medieval Jewish mystic, Casaubon sounds the depths of one occult organization after another; numerologists, Illuminati, astral projecters, synarchists, Rosicrucians, New Agers, channelers, Freemasons and other devotees of the “tellurian currents” all come under Casaubon’s scrutiny. Like the hero of Borges’ story “Death and the Compass,” he wanders from one strange corner to the next, on to a nicely twisted ending that bares the totalitarian fascination with secrecy, an obsession that defines the will to power. At the close of the novel, the innermost mystery still unresolved but lots of mayhem accomplished, we’re still not quite sure which conspiracy will prevail—or which reality actually governs the world.

What we do know is that it’s a dangerous business toying with authors. Fans of intellectual mysteries will find no more pleasing guide to the tellurian currents than Foucault’s Pendulum, 25 years on.