For more than two years, Walter Frankenstein and his small family were among the estimated 6,500 human U-boats in Berlin — Jews trying to elude Nazis by figuratively hiding like submarines. They took refuge in bombed-out buildings, cars, forests, craters, brothels or wherever they could survive for another day or week.

One morning in 1944, after sleeping in the shell of a building, Mr. Frankenstein was riding a train when a military policeman demanded to see his identification. Years later, in an interview with the Jewish Museum Berlin, Mr. Frankenstein recalled that he told the officer, in a fake foreign accent, that he was a forced laborer and had left his papers in his work clothing.

When the officer insisted on calling his employer, Mr. Frankenstein felt he had no choice but to admit that he was a Jew, although he risked being deported to Auschwitz. But the officer did not report him. Instead, he told Mr. Frankenstein: “Get lost. I’m not looking for Jews; I’m looking for deserters.”

That episode illustrated the daily threat faced by Jews in hiding — sometimes in plain sight — during the Holocaust, and the luck that kept some of them alive.

“The average U-boat changed locations on average one dozen times during the war,” Richard N. Lutjens Jr., a professor of modern German history at Texas Tech University and the author of “Submerged on the Surface: The Not-So-Hidden Jews of Nazi Berlin, 1941-1945” (2019), said in an email. “They had to. The constant air raids and suspicious neighbors meant that one would rarely stay in one place for too long.”

Mr. Frankenstein, who was one of about 1,700 such U-boats who survived the war, died on April 21 in Stockholm, where he had lived since 1956. He was 100.