Serving aboard an American submarine was one of the most dangerous assignments in World War II, with nearly 1 in 5 crew members losing their lives somewhere in the ocean depths.

Paul Wittmer of suburban St. Louis has spent years working to ensure that those men — more than 3,600 sailors — are remembered, including a book that has been years in the making.

Wittmer, a submarine veteran who recently turned 90, has conducted research for eight years at the National Archives at St. Louis, which houses millions of military personnel records. He compiled biographical information on every man lost aboard a submarine during the war. The research fills six volumes.

The painstaking effort even helped correct history. The Navy previously listed 3,505 submarine officers and sailors lost on 52 subs downed during World War II. Wittmer calculated a larger number — 3,628.

The duty was highly risky, Wittmer said, and about 20,000 men volunteered.

“When you go on a patrol, you are essentially alone,” Wittmer said. “You didn’t have any support group, and you went deep into the enemy harbor. You rescued people. You plotted enemy mine fields. That was a very nasty business.”

Wittmer has been active in submarine veteran organizations, helping to get monuments erected to honor the dead. For years, he wanted to compile their life history in a book, but much of the personnel information wasn’t available to the public. Wittmer tried Freedom of Information requests, but to no avail.

In 2007, the government made public personnel records for all veterans discharged as of 1945, the end of World War II.

“Mr. Wittmer was right there at the door waiting for us,” said Whitney Mahar, the archives’ research room manager, who says 10 to 20 people show up each day to do research. “He’s very persistent, very serious about his research and what he’s trying to accomplish.”

Wittmer poured through thousands of documents, compiling information such as the name of each veteran, his date of birth and birthplace, parents’ names, service dates and dates — or approximate dates — of death. In some cases, he was able to find photos of the men, who came from all across America.

Wittmer, who grew up in New York City, joined the Navy in 1942. Curiosity led him to enlist on a submarine.

“They had the best diesel engines, and I wanted to know about diesel engines,” he said. “I actually learned quite a bit — how to operate them and take them apart and put them back together again.”

After the war, Wittmer worked as an engineer in New Jersey and Connecticut. He moved the family to St. Louis in 1978 to work at Ferguson Machine Co.

Five years earlier, a fire at the archives destroyed records of thousands of servicemen, including about 80 percent of Army personnel discharged between 1912 to 1960 and countless Air Force personnel discharged from 1947 to 1964.

Fortunately for Wittmer and families of submarine veterans, the Navy records were intact.

The latest edition of Wittmer’s self-published, six-volume set of the hardbound books, “United States Submarine Men Lost During World War II,” was published earlier this year. It is co-authored with Charles Hinman, curator of the USS Bowfin Submarine Museum in Hawaii.

The set is available for $300. Wittmer said the information in the books will not be posted on the Internet. Wittmer has sold 11 sets and donated one to the records center. He would like to get one in every state library and archive.

It wasn’t easy, he said, but it needed to be done.

“It was labor-intensive,” Wittmer said. “It was the equivalent of a 40-hour week job at my personal expense. But it was a labor of love, really.”