We know from the movies that there's no basement under the Alamo, but the Lincoln Memorial in Washington is a different story. Construction on the memorial started in 1914, which began with digging 40 feet into the soft, reclaimed ground to lay a foundation. That foundation consisted of columns three stories tall to support the edifice above. The area is known as the Lincoln Undercroft.
The underground cathedral of concrete pillars was then simply forgotten about until renovations in 1975. According to the Washington Post, in preparation for the Bicentennial, the memorial’s bathrooms were renovated, and the construction crews started peering into the building’s foundation. They brought along their friends, some of whom belonged to the National Speleological Society. The cellar was deemed a cave, complete with stalactites and its own ecosystem (insects, rodents, etc).
One other interesting find was historical graffiti from way back in 1914. Steven Schorr got to explore the Undercroft as a part of a digital preservation project, and described to NBC how “down in the basement of the Lincoln Memorial, they actually have things written on some of the pillars. The builders actually drew cartoons and they have them covered in Plexiglas.”
The basement was closed to the public again in 1989. Take a video tour, or you can visit the Lincoln Undercroft yourself in 2022, when it reopens for the memorial's centennial celebration. -via Metafilter
(Image source: Library of Congress)