When the U.S. military first considered building a presidential bomb shelter during World War II, the last place they wanted to put it was in the White House itself. This was a decade before Harry Truman had the place gutted to the bones and rebuilt in steel, and Franklin Delano Roosevelt’s executive residence was a creaky old fire trap in danger of collapse.

In the event of an aerial attack on Washington, D.C., the plan was to whisk the wheelchair-bound president across the street to the U.S. Treasury.

Within days of the attack on Pearl Harbor, work began on an inclined tunnel leading from the White House to a dry moat surrounding the Treasury. From there it was just a few feet to the Treasury basement, and safety.

The Treasury is built like a tank, with immense granite foundations and sturdy barrel vaulted cellars. One of the abandoned vaults down in the basement was hurriedly transformed into a 10-room apartment suite, complete with a command center and living quarters for the president. A Treasury Historical Association newsletter recalls that “carpeting and wall drapes were installed to make the vault a bit more habitable, and food and water supplies were stockpiled for the President’s and his staff’s use.”

The Treasury shelter was only a temporary affair while more permanent construction was taking place elsewhere on the White House grounds. During 1942, a 1,600-square-foot bunker was nearing completion, with 7-foot-thick walls to protect against 500-pound bombs. To cover construction, a two-story extension was added to the White House directly on top of the bunker. Today that’s the East Wing.