Weeks before D-Day, MI5 interrogated a school headmaster whose Daily Telegraph crosswords contained the words Omaha, Utah and Mulberry, three of the most classified code names of the war. It was pure coincidence, but one of those words, Mulberry, is the subject of this episode: the audacious plan to build two complete deep-water harbours in Britain and tow them across the English Channel to Normandy. Because the Dieppe raid had proven that capturing a fortified French port was impossible, the Allies decided to bring their own.

We break down the strange vocabulary of the project, from the sunken Gooseberry blockships and 6,000-ton concrete Phoenix caissons to the wave-breaking Bombardons, the tide-riding Spud pierheads and the flexible Whale roadways. We cover the stealth hydrographic surveys off enemy beaches, the Scottish storm that decided which roadway design survived, Captain Edward Ellsberg’s rescue of caissons stuck in the mud, and the June 19 gale that destroyed Mulberry A at Omaha while Mulberry B at Arromanches became Port Winston, operating for ten months and landing over two million men.

• Why the Allies needed 12,000 tons of cargo landed every day and why no French port was available
• How hollow concrete blocks weighing 6,000 tons floated across the Channel at walking pace
• The head-pressure pumping crisis that reached Winston Churchill’s desk before the invasion
• Why the storm annihilated the American harbour but left the British one repairable
• How American beach landings later outpaced the surviving harbour, and why Mulberry still mattered as an insurance policy