BIGOT was a code word within a code word, a security classification beyond Top Secret. When planners adopted Neptune as the code word for the naval and amphibious aspects of the invasion, they realized that greater protection had to be given to any document or map that even hinted at the time and place of D-Day. They chose the odd code word BIGOT by reversing the letters of two words—To Gib—that had been stamped on the papers of officers going to Gibraltar for the invasion of North Africa in November 1942.

Those who were to get date-and-place information were given special security back-ground checks. If they qualified, they were described as "Bigoted." So when Eisenhower learned about the catastrophe off Slapton Sands, he wanted to know whether any of the dead or missing had been Bigoted. About ten men had. Those bodies were found and their documents collected. But had the Germans found any secrets on other bodies?

Allied code breakers, who eavesdropped on German communications, listened for days to determine whether the Germans had gained any new intelligence about D-Day. They had not. D-Day's secrets were still safe.

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.