In a part of Manhattan booming with trendy green high rises, renovated lofts and digital media companies, a hidden trove of musical relics has been growing for over 30 years.

Housed in a nondescript building in TriBeCa is the Archive of Contemporary Music, a nonprofit founded in 1985. It is one of the world’s largest collections of popular music, with more than three million recordings, as well as music books, vintage memorabilia and press kits. For point of comparison, the Library of Congress estimates that it also holds nearly three million sound recordings.

Inside its space on White Street, there are shelves upon shelves upon shelves of vinyl records and CDs. Signed Johnny Cash records hang close to nearly 1,800 other signed albums. There are boxes of big band recordings, world music and jazz and original soundtracks. Most of the inventory is stored in the basement below.

Notably, the archive, which still receives about 250,000 recordings a year, is home to a majority of Keith Richards’s extensive blues collection. (Mr. Richards, of the Rolling Stones, sits on the board of advisers.)