In a story which depends heavily on architectural metaphor, the fissures that began to snake across the masonry of Gianlorenzo Bernini’s twin bell-towers at St Peter’s in Rome, even before they were complete, were highly portentous.

Resting on bases which already formed part of Carlo Maderno’s façade, the twin campanili had been stretched up to three storeys during construction and elaborated far beyond Bernini’s original design. The continuous changes were instigated by Pope Urban VIII, the architect’s patron and not a man to be crossed. Naturally, he took no blame, and the alarming cracks were charges to be laid at Bernini’s feet.

Before measures could be taken to look into the problem, the Pope was dead. Almost immediately, his successor, Pope Innocent X, began investigations. These concentrated on what was at fault below ground, whether swampy terrain or inadequate foundations. Early in 1645 Francesco Borromini, seeing the two deep shafts being dug