It’s believed that an extensive tunnel system connects the White House to other key government buildings, though the total length remains classified. One confirmed tunnel runs for 761 feet and connects the East Wing to a secret basement of the adjacent Treasury Building. There are also rumors of tunnels connecting the White House to the Capitol, the Blair House (also known as the President’s Guest House), Number One Observatory Circle (the official residence of the vice president), the Pentagon, and even Camp David — but these are all unsubstantiated for now.
https://historyfacts.com/us-history/article/whats-hidden-beneath-the-white-house/
Like Site R, Mount Weather, Greenbriar, and probably some classified sites that don't end up on Google.
Mt. Weather is logical, but that would mean a really long tunnel and a lot of people to keep the secret. But it wouldn’t be the first time.