The D-Day invasion faced an enormous logistical problem: the Allies needed to land hundreds of thousands of troops and millions of tons of supplies, but the nearest deep-water port — Cherbourg — was heavily fortified. The disastrous Dieppe Raid of 1942 had proven that a direct assault on a fortified port was suicidal.

Winston Churchill's solution, scrawled in a memo in 1942, was characteristically audacious: "They must float up and down with the tide. The anchor problem must be mastered. Let me have the best solution worked out. Don't argue the matter. The difficulties will argue for themselves."

Building the Impossible

The result was the Mulberry harbours — two complete artificial ports, each the size of the port of Dover, built in England and towed across the English Channel. Each consisted of:

Phoenix caissons — massive concrete structures, some 200 feet long, that were sunk to form breakwaters. Over 200 were built in secret at locations around the English coast.

Bombardons — floating steel breakwaters that dampened waves beyond the solid structures.

Spud piers — floating roadways supported on steel legs that rose and fell with the tide, connected to pierheads where Liberty ships could unload directly.

The Great Storm

Mulberry A (at Omaha Beach) and Mulberry B (at Gold Beach, called "Port Winston") were both operational by June 19. Then, on June 19-22, the worst Channel storm in 40 years struck. Mulberry A was destroyed. Mulberry B survived because its position was more sheltered and because its caissons had been more firmly seated.

Port Winston operated for ten months. Through it passed over 2.5 million men, 500,000 vehicles, and 4 million tons of supplies. Remnants of the concrete caissons can still be seen off Arromanches beach today, slowly being consumed by the sea — one of the most remarkable engineering achievements of the war, hiding in plain sight.