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.”
Know Before You Go
Approximately three quarters of the way up the staircase, there is a doorway, that upon opening, leads you out into a 360 degree walkway inside the Rotunda. You are just below the large paintings on the inside of the dome, looking down on all the tourists, etc. This walkway was the primary destination for 99 percent of the dome's visitors, in the past. Additionally, there is another small doorway that opens to the outside 360 degree walkway, on the outside of the dome. As you can imagine, the view from this spot is spectacular. This outside walkway is still occasionally used as a good viewpoint for official photographers, and other official government entities.