We think we know most of what there is to know about the world’s famous buildings, but some unusual features are hiding in plain sight. Sometimes, that’s by design, such as when it comes to hidden amenities or spaces for the rich and famous. Other secrets are buried beneath the surface — in at least one case, quite literally. A few are just bizarre, like the European monument that’s secretly a giant telescope. Do you know all the secrets of these six buildings? Even if you can’t see these features on your next trip, they’re still fun to think about.

The Eiffel Tower Has a Secret Apartment on Top

Apartments with Eiffel Tower views are highly sought after — but what about an apartment with a view from the Eiffel Tower?

The iconic Parisian landmark was designed by bridge engineer Gustave Eiffel’s firm for the 1889 International Exhibition. Even though it was designed for a huge public event, Eiffel left a little treat for himself at the very top: a 1,076-square-foot apartment with a wrap-around balcony that he famously did not let anybody else stay in (although he would occasionally entertain high-profile visitors, including Thomas Edison). When the tower was built, it was the tallest human-made structure in the world, making the abode that much more luxurious.

The studio apartment was outfitted with a table, couch, piano, and three small desks (as well as a kitchen and bathroom facilities), and Eiffel supposedly never slept there, preferring to use it as an office where he could tinker with scientific experiments. It’s open to the public now, and staged with wax figures of Eiffel, his daughter, and Edison.

The Waldorf Astoria Hotel Has a Secret Train Station

The Waldorf Astoria is one of New York City’s most iconic luxury hotels, with many high-profile galas and celebrity guests since it was built around 1930. It’s a popular destination for dignitaries, including many United States Presidents — a few of whom took advantage of the hidden train station in the depths below.

The most famous user of the secret train station was Franklin D. Roosevelt, who escaped from the platform via his private train car in October 1944 so the public wouldn’t see his wheelchair. Generals of the Army John J. Pershing and Douglas MacArthur both made use of it, too. And Andy Warhol once threw a big party down there. Apparently it’s still available for visiting Presidents wishing to make a speedy exit.

The Washington Monument Has a Tiny, Underground Twin

When the Washington Monument was in its last phase of construction in the late 1880s, it had a puzzling little structure at its base: a scale replica, just 12.5 feet tall. The mini-me helped surveyors calibrate their equipment and ensure their readings were accurate as they measured the topography around the area.

Soon after the monument’s completion, the whole area was graded (meaning it was landscaped to be level), which involved burying the base of the monument past the height of the miniature. The smaller monument was encased in brick with a utility cover on top.

While the mini monument is not necessarily common knowledge, it’s well known to government surveyors, who still use it as a geodetic control point, one of some 1.5 million such markers across the country.

The Real Taj Mahal Sarcophagi Are Hidden

The Taj Mahal, built in the 17th century in what’s now India, is an architectural marvel — it’s packed with clever optical illusions, and even changes color at different times of the day. Mughal Emperor Shah Jahān had it built after the death of his most beloved wife, Mumtaz Mahal, as both a memorial and a tomb — and although he reportedly planned on building himself a matching one, it ended up being his final resting place, too.

Two structures that look like sarcophagi sit in the center of an eight-sided room decorated with semiprecious stones and a marble lattice, but they’re just cenotaphs for show. The actual sarcophagi are resting below. Shah Jahān’s cenotaph is slightly off-center, possibly since he hadn’t actually planned to be buried there; it’s the only thing that throws off the building’s otherwise perfect symmetry.

The Monument in London Is Secretly a Telescope

London’s The Monument, a 202-foot tower that looks like a pillar topped with a flaming orb, commemorates the Great London Fire of 1666. Visitors can climb more than 300 steps up a spiral staircase to see the view from the top. But it’s not what it appears.

Its design is often attributed — even on its own plaque — to famed architect and astrologer Christopher Wren. However, it was actually the brainchild of his friend Robert Hooke, a wildly influential scientist who coined the word “cell.” Hooke was tasked with building a monument, but he also wanted a giant telescope, so he ended up combining the two. The top end of the telescope is the orb, which opens up to let in the night sky. The bottom end is through a hatch below the tower, in Hooke’s former physics lab. When both the orb and the hatch are open, you can look upward from the basement lab to view the night sky.

The telescope wasn’t actually feasible with the technology available at the time, because the lenses were destabilized by traffic vibrations from the busy road outside. It still worked out for Hooke, though — there weren’t a lot of high buildings at the time, so he ended up using it to study atmospheric pressure.

The Leaning Tower of Pisa Is a Big, Empty Tube

The Leaning Tower of Pisa was built in multiple stages between the 12th and 14th centuries. The first three stories were built before the foundation settling in the soft soil that caused the tilt was noticeable. A century later, five more stories were built on top of those already-tilted three stories, attempting to correct the lean — but making it lean further. Its enduring stability despite its dramatic pitch has made it a major landmark popular with tourists… but what’s actually in there?

The answer: It’s a big empty tube, with no floors, no decoration, and no windows. Its original purpose was a bell tower, but the bells were eventually removed to help keep the tower stable. Tourists can walk up a spiral staircase along the tower’s walls to a view deck at the very top, but there is quite literally nothing to see inside.