The Jewish teenager apparently covered up the pages because she anxious that other people in her hideout would read them, as they contain a series of dirty jokes and her thoughts on sex education, said Ronald Leopold, director of Anne Frank House in Amsterdam.
Researchers have uncovered two pages of Anne Frank's diary masked over with brown paper that contain jokes and musings about sex, prostitution and contraception.
"Anyone who reads the passages that have now been discovered will be..."
Regarding sex, Frank talked about how when a young woman gets her period around the age of 14, it is a "sign that she is ripe to have relations with a man but one doesn't do that of course before one is married".
The Frank family, originally from Frankfurt in Germany, had left for the Netherlands when the Nazis gained control in the 1930s. "They make it clear that Anne, with all her gifts, was above all an ordinary girl", he added, according to the Associated Press.
The pages, dated to September 28, 1942, were contained in the red-and-white checkered diary Anne had received for her birthday in June of that year, shortly before they went into hiding. He edited and published her writings in hiding after the war, making Anne Frank one of the world's most famous Holocaust victims.
The Anne Frank House photographed the pages with a high- resolution camera and a light shining on them during a regular check on the diary's condition in 2016.
"The only element that might be interesting from the point of view about her development as a writer and as a teenager is the fact that she's creating, kind of, fiction" he said. Imagining that someone might ask her about "sexual matters", Frank tries to provide answers to an imaginary friend.
Cooped up in an attic for months on end, Anne Frank jotted down some of her observations about life in her diary.
The pages' content had remained unknown for decades because Frank had erased them.
"She was probably afraid that other people she was hiding with, either her father, her mother or the other family would discover her diary and would read these things", Leopold said. That the pages existed was already known, but she had pasted the two original diary pages with brownish paper.
Miss Frank and her sister died in camp, when she was just 15. Only Anne Frank's father, Otto, survived the Holocaust.
Her diary was found after the war and would become one of the most read books in the world.