For more than 70 years, two pages of Anne Frank’s diary have remained a mystery. Until now.

For more than 70 years, two pages of Anne Frank’s diary have remained a mystery.

The Jewish teenager, who kept her diary while she was in hiding in Amsterdam during WWII, stuck adhesive brown paper over those two pages, to cover them up. Her diary was published after the war and went on to sell more than 30 million copies worldwide. But no one knew what was written under the brown paper – until now.

New photographs of the pages were taken in 2016, and researchers used image-processing software to finally decipher Anne’s words. So what was it that she wanted to hide?

Four “dirty” jokes, as she called them, and an explanation of sex and prostitution.

“Anyone who reads the passages that have now been discovered will be unable to suppress a smile," Frank van Vree, director of the Netherlands Institute for War, Holocaust and Genocide Studies, tells ABC.

“The 'dirty' jokes are classics among growing children. They make it clear that Anne, with all her gifts, was above all also an ordinary girl.”

Anne was just 13 when she wrote those pages of her diary. It was September 28, 1942, less than three months after she and her family had gone into hiding from the Nazis.

“Do you know why the German Wehrmacht girls are in Holland?” she wrote.

“As mattresses for the soldiers.”

Another joke has a man coming home and becoming suspicious that his wife has been in bed with another man. He searches the house and finds a naked man in a closet.

“When one man asked the other man what he was doing there, the man in the closet said, ‘Believe it or not, I’m waiting for the tram.’”

Anne writes about sex as if she’s giving an explanation to someone else. After going into details about periods, intimacy and contraception, she gives a description of prostitution.

On prostitution, she writes: “All men, if they are normal, go with women, women like that accost them on the street and then they go together. In Paris they have big houses for that. Papa has been there.”

Ronald Leopold, executive director of the Anne Frank House tells Reuters why Anne covered these two pages.

“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,” he says.

Anne, her parents, Edith and Otto, and her older sister Margot spent more than two years in a “secret annexe” above Otto’s Amsterdam business premises during WWII, along with the van Pels family and another man. The Franks had tried to emigrate to the US, but that possibility was blocked to them.

Otto’s employees risked their lives to hide them and keep them fed.

In August 1944, German police stormed the annexe. The Franks and the others hiding with them ended up in the Auschwitz concentration camp. Anne’s mother Edith died at Auschwitz, while Anne and Margot died at another camp, Bergen-Belsen, in early 1945. Otto survived.

When Otto returned to Amsterdam, one of his former employees gave him Anne’s diary, which she had found. He managed to get the diary published in 1947, and it became one of the world’s most popular books, translated into more than 60 languages.

Otto’s initial version left out certain sections of Anne’s diary, including those that criticised her parents and talked about sex. These were restored to later editions.

As for the final two covered pages, Leopold says on the official Anne Frank website the decision to publish them was made due to “great public and academic interest”.

"They bring us even closer to the girl and the writer Anne Frank."