The Dominican church and under
Napoleon's soldiers are known to dismantle coffins for firewood here, urban legends claim that these remains are of French soldiers from the 1812 French invasion of Russia or of victims of the Inquisition or plague. More romantic are claims that these dungeons were once part of a larger tunnel network that allowed legendary lovers Barbara Radziwiłł and Sigismund II Augustus to meet in secret.
Please note: not available to public as of July 2019.
Read more: https://en.wikipedia.org/wiki/Dominican_Church_of_the_Holy_Spirit