Library of Congress

The largest library in the world, with over 170 million items, housed in three buildings behind the Capitol. The one to see is the Thomas Jefferson Building of 1897, an exuberant Beaux-Arts palace with a Great Hall of marble, mosaics and murals that many consider the most beautiful public interior in the United States. From the overlook you see the octagonal Main Reading Room under its dome. Exhibits include a Gutenberg Bible and the contents of Thomas Jefferson's personal library. Free; timed passes are required for entry and can be booked online. An underground tunnel connects it to the Capitol Visitor Center. Allow 45 minutes to an hour.