By all external appearances, the walls of the dome on the U.S. Capitol building are a solid marble edifice. But the shell is actually hollow, made out of metal (painted white), and supported like an iron-framed skyscraper. The tight space between the exterior facade and the interior walls contains a stairwell with 365 steps.
Up at the tip-top of the dome the stairs lead to a small room known as the Tholus. This round lookout spot is the vertical projection you see in between the circular bit of the dome and the Statue of Freedom. The government has released several photos over the years from the Tholus looking out, but there’s only one mysterious photo floating around (from Wikimedia, of all places) that shows the interior of the Tholus. Based on architectural plans from the 1860’s, it’s a simple affair, but it would still be interesting to catch a glimpse.
The familiar present-day structure is actually the Capitol building’s second dome. The original so-called Bulfinch Dome was smaller, made of wood and clad with copper. Work began on the second, more vertically gifted dome in 1855 and was completed in 1863 during the American Civil War. Iron was selected as the building material because it was fireproof, as well as cheaper and lighter than stone.
The interior of the dome was once open to the public, but according to the U.S. Capitol Historical Society, “so many persons collapsed and had to be carried down, and so much trash was tossed below, that the area was closed.”