If you’ve ever been to the Lincoln Memorial in Washington, D.C., you’ll know it’s a majestic site, with the 19-foot-tall Georgia white marble statue of Honest Abe overlooking the Reflecting Pool and then further off, the Washington Monument and the U.S. Capitol.

Something you presumably didn’t see on your visit, however, is a massive 50,000-square-foot foundation built to keep the whole thing from sinking into the swamps of D.C. It’s called the Undercroft, and it’s been a tightly held secret for decades. In June, the Department of the Interior will invite the public to see the hidden vault, Secretary Doug Burgum announced Sunday: