Smack dab in the middle of the New York Metropolitan Area, and right next to Riker’s Island, North Brother Island is a small and uninhabited mess. Now officially a bird sanctuary, it used to house the Riverside Hospital for quarantinable diseases, and was bought back by the government in 2007. Nobody is allowed entry without a permit.

Lake Eerie might be a beautiful (but cold) lake linking Buffalo to Cleveland to Toledo, but it hides a deep secret. Under the lake, there’s a massive three-mile-long salt mine so deep that workers must carry their own oxygen, while more is pumped in. It also is full of abandoned shafts and passageways.

Eiffel tower designer Gustave Eiffel decided to throw a little perk into his creation, and included a private apartment for himself atop the structure. Although it's hard to imagine where it would fit from the outside, it’s there.

Beneath LA sits a massive network of mostly abandoned tunnels. Used by everyone from bootleggers to government officials, the tunnels are mostly off limits to the public… unless you can find a way in.

When Mount Rushmore was under construction, designer Gutzon Borglum imagined an interior chamber filled with American history and documents. But when the project went unfinished, and Borglum died, the chamber was sealed.

Above the hustle and bustle of New York City’s Grand Central station, sits the Vanderbilt Tennis Club. Does it make sense to use that valuable space in that way? Not really, but it’s been there since the 1960s.

Smack dab in the middle of Disney World in Orlando, Florida, sits an abandoned island. The island has been abandoned for over 25 years, but before 1999 it was known as Discovery Island, and was Disney’s precursor to the Animal Kingdom. However, it is now an overgrown mess.

The Leonardo daVinci statue was unveiled at Rome’s airport in 1960, but a key feature remained unknown for another 46 years. In 2006, while undergoing renovations, workers found a secret compartment and two parchments. The parchments detailed the area's history, and the statue’s construction.

Mount Weather is currently the center of operations for Fema. But back during the cold war, it was a relocation site for high ranking officials in the case of a catastrophic natural or nuclear disaster. Although regular government operations took place above ground, its second function and location inside the mountain was unknown until TWA Flight 514 crashed into Mount Weather in 1974.

The Cheyenne Mountain Complex is a United States Space Force installation and defensive bunker in El Paso County, Colorado. The mountain was excavated for a command center during the height of the Cold War in the 1950s, and it now houses the advanced North American Aerospace Defense Command, (NORAD).