In its 150 years, the Smithsonian has accrued 150 million pieces in its collection of objects, artifacts, specimens and creatures. So, where do you house all of this history? Even though the Smithsonian is sometimes referred to as “America’s Attic,” the truth is that many of the most interesting (and least visited) Smithsonian collections are in the basement.

In fact, most people don’t know that there’s a world of collections and cultural activity beneath the National Mall.

Known as the Quadrangle complex, these spaces — the Arthur M. Sackler Gallery, the National Museum of African Art and the S. Dillon Ripley Center, along with the Enid A. Haupt Garden — opened in 1987 and were constructed to enhance the Smithsonian’s focus on non-Western art. But they also hold a far greater distinction: With Mall real estate at a premium, these buildings are the first underground museums in Washington.