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The S. Dillion Ripley Centre (free entry) is one of many museum and exhibition venues located along the south side of The Mall in D.C. Opening hours are 10am to 5.30pm daily.
This is a small, circularly shaped kiosk style building with a copper dome, situated between the Smithsonian Castle and the Freer Gallery. Upon entry, you descend several floors where you find exhibitions which change throughout the year.
The underground exhibition area is directly linked to the Freer Gallery, Arthur M. Sackler Gallery and National Museum of African Art. As such, you can actually use the entry for any of these four venues and each any of the others including the Ripley Centre exhibition space.
Note: The Smithsonian Ripley Centre webpage has information on current and upcoming exhibitions. This is worth a quick review if you are considering making a stop here during upcoming visits to D.C.