sentences:  Unlike books which have gained the reputation of being "underground" after being banned, the location of books that were written to be circulated discreetly and thus avoid charges of obscenity are rarely held by the National Diet Library, so researchers make an effort to hunt down the books themselves.Even publishers of these books themselves may not know the whereabouts of any existing copies, making knowledge of these books scarce in underground book communities.In 2011, specialist and collector of banned books, donated roughly 7000 books of this nature to Meiji University.