Giving an insight into the gruesome medical experiments performed by Nazi concentration camp doctors, some 27 letters written by Polish women inmates have been given to a small museum to be preserved. The letters which had details of the experiments made on 74 women at the Ravensbrueck camp in 1943-1944, were not some spy messages but normal notes sent to their families but along with that were some invisible messages written between the lines and in the margins.

Krystyna Czyz-Wilgat was among those women who wrote several of these letters. Her family has donated them to the Poland museum though some of them are in such a poor condition that it is not clear whether the “Under the Clock” Martyr Museum in Lublin, in eastern Poland, will put them on public display.

Since every note had to be passed through sensors, these messages were written with a thin wooden stick. The urine lost its colour and became invisible after an acid reaction with the paper so to read the hidden message, recipients had to heat up the letters.

The first message that they sent in this manner also contained a clue that following letters would have secret messages written in urine. It was due to these hidden messages that the atrocities on these 74 women came to light before the end of the war in 1945. Gruesome medical experiments were conducted on them including being injected with gangrene to test new drugs.

Claiming that these letters are extremely valuable and historic, curator of the Lublin museum Barbara Oratowska said that despite there being broad reports about the Auschwitz camp, on Ravensbrueck there was little information released on the matter