The letters appear to be normal communication to family members, but, when heated up, also tell the story of deadly procedures performed on the inmates of the Ravensbrueck camp

Some 27 letters written in urine by Polish women inmates to report on gruesome medical experiments performed on them by Nazi concentration camp doctors have been given to a small museum in Poland to be preserved.

The letters, which informed the world about the deadly experiments made on 74 women at the Ravensbrueck camp in 1943-1944, were apparently normal notes to families but with invisible messages between the lines and in the margins.

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