The Piazza dei Miracoli is the brightest expression of an architectural school — Pisan Romanesque; but it entered history above all through the lesson it gave unintentionally: no building is sounder than the ground it stands on.

The cathedral and bell tower are faced in white-and-grey marble and dressed with tiers of blind arcades on slender colonnettes; this layered façade turns the heavy Romanesque wall into a light lace of light and shadow, thinned gallery by gallery as the mass rises. Yet the whole complex sits on loose, water-laden alluvial clay. Before the bell tower had even reached its third storey it began to sink to one side — the foundation settled toward the softer ground and the tower leaned.

The builders' response is the most instructive part of the work: construction was halted, decades were allowed to pass (so the ground could consolidate somewhat), and the upper storeys were then raised slightly the other way, curved like a banana to compensate for the tilt. The tower is thus not merely leaning but carries a hidden curve trying to correct its own inclination. Pisa is the canonical example of the cost of forgetting monumental architecture's invisible partner — the bearing soil — and of the ways to negotiate with it; the modern discipline of soil mechanics (geotechnics) matured partly in trying to understand and save this tower.