Among its collections, the Virtual Library of Historical Newspapers has a valuable compilation of underground newspapers: almost 90,000 pages digitised by the Directorate General for Books, Archives and Libraries under the umbrella of the Ministry of Education, Culture and Sport thanks to an agreement with the Marxist Research Foundation (FIM), which manages the newspaper archives of the Spanish Communist Party. These materials are an invaluable tool for studying the political activities of the underground groups that opposed the Franco regime, in particular the activities of the Communist Party and the organisations that revolved around it.

It should be stressed that, in many cases, these publications were produced using clandestine artisanal methods; very poor quality paper and ill-suited typographic instruments were employed, and some publications were even written and illustrated by hand. Some are in an advanced state of degradation. Thanks to digitisation, not only is the dissemination of these collections via the web ensured, but also the preservation of their content is guaranteed.

The 244 publications that make up this collection are dated from between 1932 and 1976 and were published in Spain, Algeria, Colombia, France, Italy, Morocco, Mexico, Russia, Venezuela, Uruguay, etc.