Monuments Facts

The Lincoln Memorial has a secret: a 43,800 sq ft cave beneath it - nearly twice the floor space of the memorial above. Rainwater seeping through the marble has grown real stalactites, and 1914 construction workers left cartoon graffiti on the columns, now preserved behind plexiglass. It opens as a public museum for the first time on June 25.

Mount Rushmore has a secret: a 70-foot granite tunnel cut directly behind the faces. Sculptor Gutzon Borglum began drilling it in 1938 to house America's founding documents for future civilizations. Congress shut the project down in 1939 and Borglum died in 1941. In 1998, his family lowered 16 porcelain panels into a titanium vault and sealed it under a 1,200-pound granite capstone. Nobody can go inside.

The Washington Monument is two different shades of marble. Construction started in 1848 but funding collapsed in 1854, halting work for over 20 years. When the Army Corps of Engineers resumed building in 1879, the original Maryland quarry could no longer supply matching marble. Builders used stone from a new source - and it aged to a slightly darker shade. The color line sits about one-third of the way up the 555-foot tower.

The floor of Hoover Dam's monument plaza is a celestial map. Sculptor Oskar J.W. Hansen embedded 209 brass discs into the terrazzo, each marking a star or planet visible on September 30, 1935 - the night FDR dedicated the dam. It took 200,000 astronomical calculations. If every historical record vanished, a future civilization could read that floor and calculate exactly when Hoover Dam was dedicated.

The Taj Mahal was disguised three times to protect it from bombers. In 1942, British engineers wrapped the dome in bamboo scaffolding. Without GPS, it looked like a building site from the air. India used the same trick in 1965 and 1971. In 1971, over 18,000 lbs of green jute were draped over the marble. Not one bomb ever fell.

Stonehenge's central slab is the Altar Stone - a 6-tonne sandstone block. Scientists believed for a century it came from Wales. A study in Nature matched its mineral fingerprint to northeast Scotland, over 750 km away. That is the longest recorded stone haul of the Neolithic era. How Neolithic people moved it there remains unsolved - possibly by sea.

The Gateway Arch was built as two separate legs measured to meet precisely at the top. On October 28, 1965, the morning sun expanded the south leg. The last piece would not fit. Workers ran fire hoses to the top and soaked the steel until it shrank. Hydraulic jacks applied 450 tons of force, and the final section slid in.

The Lincoln Memorial hid a secret room for over 100 years. It just opened to the public for the first time. The new $69 million museum has floor-to-ceiling glass walls. Visitors can walk past the columns holding up the memorial above. Lincoln's own signed Emancipation Proclamation and 13th Amendment are inside.