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, Frankenstein was riding a train when a military policeman demanded to see his identification. Years later, in an interview with the Jewish Museum Berlin, 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, 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 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.”

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

His death was confirmed by Klaus Hillenbrand, the journalist who turned an article he wrote about the Frankenstein family for a German newspaper into the 2008 book “Nicht Mit Uns” (the title means “Not With Us”).

Frankenstein was born June 30, 1924, in Flatow, Germany (now Zlotow, Poland). His father, Max, owned a general store that his mother, Martha (Fein) Frankenstein, ran after her husband died in 1929. The family lived above the store.

Like other Jewish businesses, the family’s store was boycotted; someone also fired bullets into it. The strangulation of Jewish life under Nazi rule meant that Jews were banned from public schools; when Walter was expelled from his school in Flatow, his mother sent him to live at the Baruch Auerbach Orphanage for Jewish Boys and Girls in Berlin, where he could attend a Jewish school.

The orphanage, he later said, was a haven for some 200 children and teenagers. “We lived there as if on a small, sheltered island,” he told the Jewish Museum. “We didn’t have much experience with persecution until the pogrom of ‘Reichskristallnacht’ in 1938.”

During the Nazis’ coordinated antisemitic violence on Kristallnacht, or the Night of Broken Glass, storm troopers burst into the orphanage, bent on burning the building down. But Walter and three other boys managed to persuade them not to set fire to the orphanage because neighboring buildings might also go up in flames.

The Nazis turned their attention to the synagogue next door, where they extinguished the eternal flame at the altar and opened the gas valve, hoping to cause an explosion. When the boys smelled gas, they turned off the valve and flung open the windows.

Later that night, Walter and the other boys went up to the roof of the orphanage to survey the damage wrought by the Nazis. The destruction, which had occurred throughout Germany and Austria, led to the deaths of at least 91 Jews, as well as fires at thousands of Jewish businesses and synagogues.

“Then we knew: The synagogues were burning,” Frankenstein said in 2018.

He met his future wife, Leonie Rosner, at the orphanage, and they left together in 1941, subletting a room in Berlin. They married in 1942; Walter, who was only 17, needed his mother’s permission.

To support themselves, he worked as a mason in Berlin, which brought him into contact with Adolf Eichmann, a pivotal architect of the Final Solution, who threatened him as he did plastering work in Eichmann’s official residence.

“One speck and you’re in Auschwitz tomorrow,” he recalled Eichmann saying.

Frankenstein’s time underground began in February 1943, when he showed up for a forced labor assignment. After a Gestapo official told him that the other Jewish workers had been deported the night before, Frankenstein fled, ripping off the yellow star that Jews were forced to wear.

He went into hiding with his wife and their son, Peter-Uri, who had been born the month before.

For the next two years and two months, the family — which grew with the birth of a second son, Michael, in September 1944 — eluded the Nazis.

At one point, Leonie and Peter-Uri escaped to a farm in Briesenhorst, hundreds of miles away, near the Baltic Sea. He took refuge in an opera house, various theaters, abandoned cars and the home of a Christian woman.