You might be fooled into thinking you know everything about these iconic U.S. landmarks, but despite their international fame, there’s more to these hotspots than meets the eye. There are hundreds of hidden features inside our nation's most renowned sites that only the most dedicated visitors will ever find. Here are a few of the most interesting and mysterious.

A Hidden Chamber Sits Behind Mount Rushmore

While sculptor Gutzon Borglum never came close to realizing his original vision for Mount Rushmore — the presidential heads were meant to top presidential bodies — he did get started on another little-known component of this South Dakota memorial. Nestled in a small canyon behind Abraham Lincoln's head is the entrance to what was intended to be an 80-by-100-foot "Hall of Records," which was supposed to be stocked with copies of historical documents such as the U.S. Constitution and busts of important Americans.

Borglum was hopeful the Hall of Records would explain the significance of the memorial to future civilizations, but he was ordered to stop its development after only a year, because Congress wanted him to focus on the construction of the faces. The combination of his death in 1941 and America's entry into World War II ended any hope of the hall’s revival. In 1998, 16 porcelain enamel tablets containing documents such as the Declaration of Independence and records describing Mount Rushmore's creation were placed in a vault deep in the unfinished chamber, apparently only for future civilizations to peruse since the chamber remains off-limits to contemporary tourists.

The U.S. Supreme Court Building Houses a Basketball Court and Gym

Located a floor above the "Highest Court in the Land" is a court of a very different sort, where justices who normally hear arguments over the gray areas of the American legal system instead argue over who fouled whom. Initially designed as a storage space, this area was converted into a gym in the 1940s, with a pair of backboards installed to accommodate what became the in-house favorite sport. Adjacent to a full weight room, the smaller-than-regulation-size basketball court mainly sees games played by clerks and off-duty security officers, but has also drawn participating justices. Activity in this part of the building apparently gets intense enough to have necessitated a sign reading: PLAYING BASKETBALL AND WEIGHT LIFTING ARE PROHIBITED WHILE THE COURT IS IN SESSION.

Philadelphia's William Penn Statue Has an Entry Hatch Located on Its Hat

Unlike most of the other landmarks noted here, the statue of William Penn is almost exclusively viewed from a distance, due to its location atop Philadelphia’s City Hall. As such, few people have intimate knowledge of its features, which include an entry hatch on the hat of the city's famous founder. Those who do get inside the hollow structure can see the chalk-carved names left behind by previous workers, or take a closer look at the flange-bolting craftsmanship that pulled the statue's 47 distinct parts together. Unfortunately, there aren't any tours that wind their way to the inside of Billy Penn, and the hat access is mainly used by the conservationists who deal with cleaning the 37-foot, 53,000-pound bronze behemoth every decade or so.

A Pristine Penthouse Endures Inside Radio City Hall

When the lavish, art deco-styled Radio City Music Hall was built in midtown Manhattan in the 1930s, an equally lavish penthouse was carved out for the theater's flashy impresario, Samuel "Roxy" Rothafel. Boasting cherry-paneled walls, gold-leaf ceilings, custom-made furniture, and a dome-shaped dining room, the penthouse was as much a refuge for relaxing as it was a stylish venue in which to entertain visiting luminaries such as Judy Garland and Walt Disney.

Rothafel himself only got to enjoy the luxury for a few years until his death in 1936, and the apartment lay mostly untouched in the decades that followed. In recent years, it's been cleaned up for tours, with the original furniture and fixtures on display, while high rollers who want to celebrate like it's the end of Prohibition can even rent out the space for special occasions.

Niagara Falls Can Be "Turned Off"

OK, there isn't a giant faucet that can be twisted to halt the 3,160 tons of water that gush over Niagara Falls every second, but portions of the world-famous falls can be dried up by the construction of cofferdams to divert the flow of water. This happened in the 1950s on the western flank of Horseshoe Falls, on the Canadian side of the border, in an attempt to reshape the rock face and alter the "presentation" of the cascading water. Another cofferdam was erected in 1969 to investigate the rock debris that had piled beneath the American Falls section in New York, before authorities ultimately decided to leave the rock pile in place. There reportedly are plans to turn off the falls again on the American side to replace a pair of century-old bridges that aren't used anymore, although this project has remained in limbo for several years.

Once-Submerged Attractions Can be Seen Above Lake Mead

Formed by the construction of the Hoover Dam in the 1930s, Lake Mead contains a wealth of underwater sights for the scuba divers who flock to this recreation area on the Nevada-Arizona border. And thanks to the megadrought that's plagued the Southwest since the start of the 21st century, many of these once-submerged attractions are now partially or completely visible to landlubbers. The list includes a World War II-era "Higgins boat," as well as the remains of the Mormon-founded settlement of St. Thomas. While these spectacles garner attention from those who don’t know the difference between a wetsuit and a drysuit, divers can take heart in the news that water levels seem to be on the rise again after reaching historic lows in October 2022.

The Empire State Building Has a Secret 103rd Floor

Although the Empire State Building is advertised as having 102 floors, that's not quite the case. There's actually another floor that was originally constructed for building maintenance. It now acts as an ultra-exclusive hotspot for the rich and famous.

Unlike floor 102, which features a wrap-around balcony and glass windows, floor 103 is ultra-thin with only a knee-high railing separating observers from the sky surrounding them. Inside the building, there's a small room used for housing electrical equipment, but most celebrities just stay on the balcony and enjoy the adrenaline-inducing photo op.

There's a Time Capsule at the Top of the St. Louis Arch

Most visitors to the Gateway Arch in St. Louis don’t realize that the monument also provides a look back in time. In October 1965, a time capsule was added to the top of the Arch. The contents aren’t exactly major historical relics, but they’re surprisingly sweet — the capsule contains the signatures of more than 700,000 citizens of St. Louis, many of them schoolchildren. The capsule is permanently welded to the Arch, so it will remain intact as long as the Arch stands.

Grand Central Terminal Has a Tennis Club

Roughly one million people pass through Grand Central Terminal every day, but almost nobody knows about the tennis club that exists above their heads. Founded in the 1960s by a wealthy Hungarian immigrant, the tennis club consisted of two simple clay courts and existed for the public to enjoy while waiting for their ride.

In 1984, however, Donald Trump purchased the space and redeveloped it into a luxurious locale for celebrities — charging upwards of $130 per hour to rent it out. The courts stayed under his control until 2009 when they were renovated and reopened to the public. Although it's open (it’s now called Vanderbilt Tennis), you'll still need some luck in order to find it since many employees don't even know it exists.

A Cave Lies Beneath the Lincoln Memorial

Most people who visit the Lincoln Memorial spend all of their investigative energy trying to find the famous typo carved into the walls. However, underneath the memorial, there's an even better-hidden gem — a full cave complete with stalactites. Construction workers stumbled upon the cave in the 1970s when digging out an elevator shaft for disabled visitors.

Apart from the gorgeous nine-foot stalactite rock formations, the most fascinating part of the cave is the graffiti that adorns the 122 supporting columns. Visitors who embark on a cave tour can view these original World War I-era scribbles made by an unidentified construction worker. They include caricatures of everyone from Woodrow Wilson to the monument's construction foreman.

There Are Wine Cellars in the Base of the Brooklyn Bridge

Engineer Washington Roebling had some serious business smarts when he created the Brooklyn Bridge. When he first started construction, he faced two major problems: There wasn't enough money in the city to pay for the full project and two local wineries refused to move their facilities out of the path of construction.

In a stroke of genius, Roebling solved both problems by incorporating two full wine cellars into the base of the bridge on each side. To help finance the bridge, he rented the cold, dark cellars out to local businesses who needed some extra storage and generated profit for the city until the Prohibition Era. Today, the city of New York has taken ownership of the cellars, stripping the caverns of their functionality and leaving them as a dry, empty reminder.

Disneyland Has a Secret Members-Only Club

Club 33 is an inconspicuous little room at Disneyland in Anaheim, California, but the people who enter are anything but low-profile. This exclusive clubhouse boasts an invite-only guest list where members must pay anywhere from $25,000 to $100,000 to get initiated and $12,500 to $30,000 in annual membership dues after that. Even if you do have hundreds of thousands of dollars to fork over for this exclusive experience, the waitlist is six years long and spots rarely open up for new members.

On top of that, the activities of the members are held in top-secret status within the park's administration, which means you never quite know what you're getting into when you sign up. There's only one thing we do know for sure about Club 33 — it's the only place in the park to serve alcohol, which means it just might be worth the investment.