Hidden chamber found under National Mall's Smithsonian Castle, dates to 1847

Work crews at the District's Smithsonian Castle uncovered a cistern dating to 1847 that was sealed for well over a century.

Workers found the liquid container last month under Jefferson Drive SW, officials with the National Park Service said in a social media post Tuesday.

The empty cistern is lined with brick, spans 9 feet in diameter and is 30 feet deep. The discovery was unearthed once workers moved a rock on top of the structure.

“It had a small stone on the top. Once we moved that piece of stone, we could see that it actually was a brick dome-shaped structure. … It’s pretty much one of the most interesting things you could hope to find under the ground,” Carly Bond, the Smithsonian's associate director for architectural history, told WTOP-FM.

The cistern was used to collect rainwater to supply to the first building of the Smithsonian Institution, Ms. Bond added.

The building, called the castle, was opened in 1855 just a few years after the cistern was built and has been closed to the public since February 2023 when its first renovation in half a century began, according to Smithsonian magazine.

“We will be preserving as much of the cistern as we can by filling it with some kind of reversible fill so that we don’t have that hazard in place underneath the roadbed,” Ms. Bond said.