On December 8, 1959, a thin concrete arch dam on the French Riviera vanished in a fraction of a second, unleashing a 40-meter wall of water that erased entire villages and killed 423 people. It is a catastrophe that almost defies comprehension.

This episode unravels the Malpasset Dam disaster through historical records, geological surveys, and engineering postmortems. We trace how a marvel of arch-dam mathematics was undone not by its concrete, but by the impermeable rock beneath it, a hidden fault line, record rainfall, and a fatal six-hour bureaucratic delay driven by economic fear.

• How the doubly curved arch dam was an astonishingly thin 6.78 meters at its base and just 1.5 meters at the rim, yet held back 50 million cubic meters of water
• Why authorities refused to open the release valves, fearing damage to the A8 high ...