The BIGOT maps and documents were created in isolated cocoons of secrecy. One was hidden in Selfridges department store in London. BIGOT workers entered and left Selfridges by a back door, many of them knowing only that they were delivering scraps of information that somehow contributed to the war effort. Others with BIGOT clearances worked on Allied staffs scattered around London and southern England. So restricted was the BIGOT project that when King George visited a command ship and asked what was beyond a curtained compartment, he was politely turned away because, as a sentinel officer later said, "Nobody told me he was a Bigot."

The system occasionally broke down. In March 1944 a U.S. Army sergeant accidentally sent a package of BIGOT papers, some containing the target date and place of the invasion, to his sister. The family was of German descent, and the sister lived in a German section of Chicago. By chance the package broke open in a Chicago post office. Postal authorities saw BIGOT and Top Secret stamped on documents and called the FBI. Investigators cleared the soldier of espionage, though he was confined to his quarters until after D-Day. The FBI put everyone who had seen the papers under surveillance. Another serious breach came in May when a U.S. major general told guests at a London dinner party that D-Day would come before June 15. He was demoted and packed off to the United States, as was a Navy captain who had blabbed too much at another party.

The strangest breach of security came from the London Daily Telegraph, whose crossword puzzles alarmed BIGOT security officers. One puzzle, on May 2, included "Utah" in its answers. Two weeks later, "Omaha" appeared as an answer. The puzzle's author, a schoolmaster, was placed under surveillance. Next came "Mulberry," code name for artificial harbors that were secretly being built in England for use off invasion beaches. Then came the most alarming answer of all: "Neptune."

This time the schoolmaster was arrested. Confounded investigators finally decided that the words had been the product of an incredible series of coincidences. Not until 1984 was the mystery solved: One of the schoolmaster's pupils revealed that he had picked up the words while hanging around nearby camps and eavesdropping on soldiers' conversations. He then passed the odd words on to his unwitting schoolmaster when he asked his pupils to provide ingredients for his crosswords.

But nothing was more secret—or more vital to Operation Neptune—than the mosaic of Allied intelligence reports that cartographers and artists transformed into the multihued and multilayered BIGOT maps. On them were portrayed details of Hitler's vaunted Atlantic Wall, a network of coastal defenses designed to repel invaders.

To discover what the Allied invaders faced, American, British, and French operatives risked their lives—and sometimes gave their lives—in the process of filling in the BIGOT maps. Revelations about Normandy's undulating seafloor came from frogmen who also got sand samples on beaches patrolled by German sentries. Such BIGOT map notations as "antitank ditch around strongpoint" or "hedgehogs 30 to 35 feet [9 to 11 meters] apart" were often the gifts of French patriots. French laborers conscripted by the Nazis paced distances between obstacles or kept track of German troop movements. A housepainter, hired to redecorate German headquarters in Caen, stole a blueprint of Atlantic Wall fortifications.

French Resistance networks passed on precious bits of information, particularly the condition of bridges and canal locks. Wireless telegraph operators transmitted in bursts to evade German radio-detection teams. Other messages got to England in capsules, borne by homing pigeons that the Royal Air Force had delivered to French Resistance agents in cages parachuted into German-occupied Normandy. Germans, aware of the winged spies, used marksmen and falcons to bring them down. But thousands of messages got through.

BIGOT maps began with information gleaned from old scenic postcards of the Normandy coast and charts from the Napoleonic era. Next came the special deliveries from the French Resistance. Then in mid-May 1944, BIGOT mapmakers asked for low-level aerial photos of the coast. Pilots, trained to fly at 10,000 feet (3,050 meters), called this wave-top flying "dicing" because they felt that in their unarmed and unarmored aircraft they were rolling the dice with death.

On May 6, Lt. Albert Lanker of the 31st Photographic Reconnaissance Squadron made the first dicing flight. Flying 30 feet (9 meters) above the Channel to slip under German radar, he turned sharply over a large dune on the Normandy coast and, at 360 mph (580 km/h), buzzed German soldiers working on defenses. Photo interpreters examined his photos and discovered that the dune was a gun emplacement, which was added to the maps.

Lt. Allen Keith, while zooming along at about 360 mph(580 km/h), hit a gull. The bird smashed through the windscreen but was stopped by newly installed bulletproof glass on brackets in front of the pilot. Spattered by blood and feathers, Keith could not see for a few seconds but wiped off his goggles and never lost control. Photos from another of the dicing missions, this one flown by Lt. Garland A. York, revealed log posts jammed in the sand, angled seaward and topped by mines. Other photos showed that the tidal flats of the beaches were studded with "hedgehogs"—steel rails welded together and resembling giant versions of children's jacks. The obstacles were designed to impale or rip open the hulls of landing craft approaching the beach at high tide.

Some air reconnaissance photographs were processed so fast that fingerprints still appear on the negatives, showing that they were snatched from the developer for circulation. "Recent wave-top aerial photo reconnaissance," says a May 29 intelligence bulletin, "reveals that practically all types of underwater obstacles may be armed with . . . mines." The dicing photos convinced planners that landing craft had to come in at low tide and discharge troops before hitting the obstacles. So mapmakers had to figure ways to display tides and beach slopes.

Head-on aerial photos of the Normandy shoreline made by the dicing flights produced eye-level views for Allied coxswains to use as they aimed their landing craft toward D-Day beaches. BIGOT artists turned the photos into paintings that showed landmarks, such as church steeples and seaside houses. One of these artists was Navy Lt. Frederick S. Wight, who later would be renowned in civilian life as a curator and historian of modern art in the U.S.

Another BIGOT artist was Navy Lt. William A. Bostick, who worked in a commandeered London apartment. He and other artists used the pilots' panoramic photos "to make watercolors of the beaches as landing craft skippers would see them as they approached." Bostick's watercolors, emphasizing terrain features and landmarks, formed a narrow band under the maps. On the back of the maps was an invasion almanac with information about sun, moon, tide, and currents from May 25 to June 21. (The precise date of the invasion was not set until after the maps were finished.)

Bostick was especially proud of an ingenious transparent overlay that showed profiles of large and small landing craft. By adjusting the sheet over a graph of the beach slope, navigators could see where their craft would run aground and what the water depth would be. "The Army called them maps and the Navy called them charts," Bostick said. "So we called them chart/maps—pieces of paper that showed the Navy where to land the Army."

While the maps were evolving, a group of intelligence officers was busy crafting the greatest hoax of the war—a spy-running operation that was not fully revealed until the 1980s. Earlier in the war, British counterintelligence officers gave captured German spies a simple choice: Be hanged or work for us. Most chose to live. Directed by their handlers, the turncoats used seemingly clandestine radios to transmit to German spymasters a mix of real and counterfeit information. The operation, run by the wryly named XX (double cross) Committee, was meant to convince the German high command that the invasion would strike at either Nazi-held Norway or at Calais, across the English Channel from Dover.