The Underworld Library designates by metonymy the section of a library where books are kept whose reading is forbidden to the public and where bad books are condemned to burn symbolically, in a place separated from the rest of the collection. Thus the librorum prohibitorum set up in 1556 under the pontificate of Paul IV which, until 1966, brought together up to 6000 works accessible to a limited number of individuals. Ask for the code.