Printing of any publications without an official censorship's control was illegal in Poland at the time of martial law (December 1981- July 1983).

Any such unofficial activity was banned and was prosecuted by law, but it didn't stopped the political opposition from so called "Solidarity" movement from trying. One of the most interesting aspects of underground printing houses activity was production of unoficial poststamps. Starting from 13 of December, in Museum of Fortifications and Weapons in Zamość, you can see an exhibition of Polish underground postservice's poststamps from the time.