Every day, boats full of tourists and commuters float by a pale patch on the wall that lines the River Thames near Britain's Houses of Parliament.

Few notice the concrete mark, or recognize it as evidence of how close London came to drowning during World War II. It is a piece of hidden history that has been uncovered by a team of professional and amateur archaeologists.

On Wednesday a group of engineers and civic dignitaries will unveil a plaque commemorating Thomas Peirson Frank, leader of a secret squad of engineers and laborers who worked night after night during World War II to repair flood defenses hit in German air raids.

As bombs fell and fires raged, teams marshaled by Frank used rubble, sandbags and finally concrete to mend breaches in the Thames wall that threatened the inundation of thousands of businesses and homes.

"It could have brought London to its knees very, very easily," said Gustav Milne, director of the Thames Discovery Program, a project that brings together experts and volunteers to explore the archaeology of London's river.

"Not just people drowning — we would have lost buildings, it would have flooded the sewers and brought up all the sewage, it would have contaminated the water supplies, cut off gas and electricity. There would have been widespread devastation and huge loss of life."

London burned during the war, but it never flooded, due in large measure to Frank, chief engineer for London County Council, and his crews.

But their story is little known — obscured first by wartime secrecy, then by gradual forgetting.

That began to change when Milne and his team noticed the large concrete patch, 9 meters (30 feet) across at its widest, in the 19th-century river wall. Chunks of the wall's granite parapet lie scattered along the muddy river foreshore nearby.

The researchers suspected the damage had been done by a Luftwaffe bomb, but the agency in charge of the river did not have any record of it.

Deep in the London Metropolitan Archives, the researchers found files revealing the truth that had been hidden from Londoners during the war and later forgotten — the river wall was hit 121 times between 1940 and 1945, 84 of them during the Blitz of September 1940 to May 1941.

The number of bomb strikes on the river was suppressed at the time so as not to alarm Londoners or alert Nazi Germany to the city's vulnerability.

London was fortunate to have Frank, a ferociously well-organized and industrious civil servant who had served in World War I and by the 1930s was warning of the city's vulnerability to floods.

When war broke out, Frank was put in charge of maintaining London's roads and utilities. He set up four depots along the river, staffed by engineers and road-repair crews, augmented by troops from the Royal Engineers.

Each time the river defenses were hit, Frank's teams were sent in, often while bombs were still falling and with little protective equipment.

"They were working in their own clothes ... in black toxic gunge," Milne said. "They didn't even have overalls provided, much less Wellington boots."

The workers were part of a civilian army — along with nurses, air raid wardens, volunteer firefighters, police and more — that kept the city running during the onslaught.

"It was a noncombatants' war and they were all fighting on the same side, to save London," Milne said.

Frank was knighted in 1942 for his work — though details of his job were kept under wraps — and later became president of the Institution of Civil Engineers. He died in 1951.

Clive Cockerton of the Institution of Civil Engineers said Frank "is very much an unsung hero. He was recognized, but within a very small circle because it was secret."

Milne said the pilot who dropped the bomb was likely aiming for Parliament. He probably didn't realize that a hole in the river wall could have inflicted far worse damage. The Germans seem not to have realized that "water is a weapon," he added.

"They never sussed that all they had to do was knock out the parapet and they could flood all of London."

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