It's said that there're dozens of thoughts lined up behind each one we’re aware of. So, what if the following places hеld a similar mystery even though it's in plain sight? Well, apparently, some of the world’s most renowned and visited locations are home to secret nooks, shrouded in a century-long mystery. But while most of them are open to the public for a minute or two of exploration, others are completely off-limits.

For instance, the secret 44th floor at 874 North Michigan Avenue in Chicago is quite an excellent example of that. This metal-clad monster, standing at 1,128 feet, might be a major tourist attraction in Chicago, IL, USA; however, it’s also home to a bunch of hidden rooms that are only accessible to those living in the building. Interestingly, there happens to be a lounge, swimming pool, and even an actual library. But all in all, that’s probably nothing when compared to NY's Railway Station and the secret wartime escape route. In fact, President Franklin D Roosevelt would often use it as his secret passageway when traveling.

Evidently, you might have thought that you know everything about these places; however, as you’ll see in a minute, these locations can still make you scratch your head in complete wonder.

20 Hidden Apartments In The NY Public Library - New York

It’s no secret that the City of the Big Apple has got one of the most impressive and unique public library systems. The NY Public Library, for instance, is the third largest one in the world, with dozens of locations scattered throughout the city of New York. Also, the world-recognized library is home to over 50 million items. Back in the 90s, Andrew Carnegie funneled millions of dollars into the creation of this unique system. Hundreds of workers were eventually hired to work and live there with their families. Their job was to take good care of the furnace. Most of these hidden places were demolished except for at least a dozen rooms that remained in the library system, inhabitat.com reveals.

19 The Small Structure In Trafalgar Square - London, England

Much to your surprise, this small structure isn't just part of London's architecture.  It has got quite an interesting purpose: inside is London's tiniest police station, atlasobscura.com reveals. This unique station was built in the early 90s when it served as a watch-post and a magnet for the city’s marchers and protestors at the time. But today, most tourists don't even suspect that this structure had a certain purpose. But frankly, it’s not really their fault that they’ve missed it: it doesn’t quite look like the typical police station anyway, so tourists can be forgiven for not taking selfies in front of it.

18 Gustave Eiffel’s Secret Apartment - Paris, France

Designed by Gustave Eiffel, the city’s most renowned and visited landmark features a small, but super cozy apartment. When the Eiffel Tower opened in the 18th century, it didn’t need much time to win the world’s wonder and acclaim; and, expectedly, Gustave Eiffel soaked up all the praise. Once the tower opened, it soon became clear that Gustave had built a secret nook – a small apartment near the top of this fabulous creation, atlasobscura.com reveals. Furthermore, the apartment was furnished in the simplest style possible - it features old wooden cabinets and chairs as well as a piano. Well, the grand piano must have created a wonderful atmosphere back in the day.

17 The Vanderbilt Tennis Club At Grand Central Terminal - New York

Best known as a purveyor of the courts, the Vanderbilt Tennis Club is located right above the actual Grand Central Terminal. Since the mid-90s, these private tennis courts have been tucked inside the train station, atlasobscura.com details, and only few people know about that. Similarly to the other secret places, these tennis courts are yet to be discovered by the massive crowds of commuters there. So next time you’re passing through the Grand Central Terminal, just look up at the building. You never know when you're going to see somebody up there playing tennis.

16 The Royal Pavilion At Stazione Centrale – Milan, Italy

Stazione Centrale – Milan’s central railway station – is usually packed with hundreds of thousands of commuters every day; however, most have got absolutely no clue that the series of closed doors they’re regularly passing past lead to the luxurious Royal Pavilion. These otherwise unattractive doors give access to one of Milan’s most impressive, exclusive rooms – the Padiglione Reale.

According to atlasobscura.com, this Royal Pavilion was initially built for Italy’s royal family in the early 90s. But even though the Italian monarchy was dismantled in the country right after WWII, the exclusive room remained there. To give you an idea of how luxurious it was, the Padiglione Reale had a couple of levels, with marble interiors, royal emblems and stylish furniture, selected by the most eminent interior designers back in the day.

15 The Secret Compartment In Leonardo da Vinci Statue - Fiumicino, Italy

Situated at Rome’s Fiumicino-Leonardo da Vinci Airport, this fabulous statue has impressed commuters ever since its creation. The giant sculpture, built in 1960, is a fantastic tourist attraction; however, its fame certainly doubled when it was revealed that there’s a hidden gem in there, waiting to be discovered. The bizarre finding, atlasobscura.com details, was a small hatch, placed at the height of about 30 feet (which is approximately in the middle of the famous statue). Once it was carefully opened, the workers were stunned to see two parchments, still in quite an excellent condition. No wonder why some people come here with binoculars – they’re just trying to see what exactly goes on in there.

14 Radio City Music Hall’s Secret Apartment – New York

Radio City Music Hall is, indeed, like a jewel on New York’s Art Deco Crown. Since its opening in the early 90s, the hall has had millions of visitors. Legend has it that the architects of the Radio City Music Hall wanted to express their appreciation for the theater’s impresario, Samuel Rothafel, so they gave him a special, exclusive gift – a fabulously decorated apartment built high up inside the Music Hall. But since Samuel’s death in 1936, the apartment hasn’t been used, atlasobscura.com says. But even though nobody has lived there since 1936, the apartment remains in a pretty good condition.

13 Flinders Street Station Abandoned Ballroom - Melbourne, Australia

Flinders Street Station is, hands down, one of the busiest railway hubs in Australia, and it serves at least a hundred thousand commuters every day. But while the station greets a ton of people on a daily basis, the old ballroom, hidden on the third floor, rarely opens for visitors. The room has been closed off since the late 90s, but there’s still a chance to peek inside it. There are special entries granted by an exclusive “Golden Ticket”, atlasobscura.com reveals, and it’s usually tucked into the visitors’ programs of the luckiest tourists. But unfortunately, such an exclusive entry is only offered on special occasions.

12 Jess Straus' Office On The 22nd Floor At Times Square - New York

Despite its fancy location, the building at One Times Square is virtually empty. Until recently, Walgreens has leased everything up through the 21st floor. Currently, though, the building remains nearly vacant, maybe except for one tenant – Jess Straus – who usually runs the New Year’s Eve celebration, atlasobscura.com reveals. By the way, Jess runs the celebration from his 22nd-floor office above which sits the New Year’s ball itself. The ball sits there all year long as it’s attached on a metal roof deck, waiting for the annual moment of glory. Today, this place is, without a doubt, one of the most famous and photographed ones in New York.

11 The Buried Remains of Little Compton Street – London, England

One of London’s busiest roads is home to a mysterious secret and even the most frequent travelers there don't know about it. This traffic island, situated in the middle of Charing Cross Road, is a remnant of London’s old days, atlasobscura.com shares. This interesting location is hidden right beneath this busy street. Dare to look down at the metal grate that covers the "island" and you’ll certainly see a Victorian street name engraved into the wall below ground level. Few know that this long-forgotten street, which bears the name of Little Compton, sits underneath London’s present-day streets.

10 The Hidden Art Deco Tunnel Underneath The New Yorker Hotel – New York

This nicely tiled Art Deco tunnel lies hidden underneath  34th Street where the New Yorker hotel now stands. Today, the hotel’s giant red sign may be one of the most photographed places in New York; however, the story behind the building is pretty much unknown. According to atlasobscura.com, The New Yorker is home to a myriad of untold secrets, one of which is the impressive Art Deco tunnel. This place, hidden underneath 34th Street, ran from the lobby of the hotel all the way to the Penn Station. Oddly enough, thousands of people walk by the busy corner of Eight Avenue without even suspecting the existence of such remarkable tunnels.

9 The Hidden Room In Mount Rushmore - South Dakota, USA

Mount Rushmore happens to be yet another major tourist attraction in the USA, but only few can see and appreciate its true beauty. Similarly to the previous locations, Mount Rushmore surprises the most enthusiastic travelers with a hidden room as well. According to dailymail.co.uk, this secret nook sits right behind the chiseled granite that resembles Abraham Lincoln’s head. This mysterious space is known as the Hall of Records and it tells the legacy of the country to future generations. Gutzon Borglum initiated the project, but sadly, he passed away before completing it. In the late 90s, the Hall work ground was revived to the extent where the chamber room was finally completed.

8 The 103rd-floor Viewing Platform, The Empire State Building - New York

The Empire State Building has always had a magnetic pull, and that’s partly due to the sweeping views of Manhattan. Realistically, most tourists rarely consider the idea to climb to the 103rd floor as they usually get to the building’s 86-th floor observation deck. All in all, the floor’s tiny balcony is quite the perfect spot for a few photos. It can be reached via a series of elevators while the access to the top of the building includes a steep metal ladder. And those, who can’t wait to gaze out to the horizon, can continue the climb to the top secret deck, dailymail.co.uk reveals.

7 The Art-filled Secret Corridor - Florence, Italy

This mysterious art-filled corridor, named after its architect, is about 1 km long and connects Pitti palace to Uffizi Gallery in Florence. This unusually long passage-way dates all the day back to the 15th century and was initially built to honor Francesco's wedding. The creation of the corridor, as stated on dailymail.co.uk, had to cut through people’s homes. Therefore, many butchers had to relocate since their stores were about to get replaced by fancy boutiques. The new jewelry stores were simply expected to provide better views and more pleasant smells for the tourists. The initial purpose of the project was to facilitate the family’s trips to and from there.

6 Waldorf Astoria's Secret Railway Station - New York

Tucked underneath the lavish Waldorf Austria hotel is a secret wartime escape route used by President Franklin D Roosevelt. This secluded platform allowed the president to travel from one place to another without being noticed by the public. This secret route was in particular help during the war years, dailymail.co.uk reveals.

Known as the Grand Central Terminal' Track 61", the President regularly used it when he didn’t want to attract any public attention. Also, his private railway car allowed him to go straight off the carriage, and into an elevator that leads to the hotel.

5 Crystal Palace Subway - London, England

Dating back to the 18th century, London’s Crystal Palace Station was a stop on the city’s High Line until 1936 when it was engulfed in flames. The arched subway lies beneath another ordinary stretch of the A2012, and only few people have heard about its existence. The old subway, dailymail.co.uk explains, also served Londoners as an air raid shelter during World War I. Although the station itself was demolished in the mid-90s, the subway was left pretty much untouched. Well, apparently, this ancient Victorian construction is yet another secret nook that nobody expects to see there.

4 Access To The Statue of Liberty's Torch, New York

If you’ve ever visited the Statue of Liberty in New York, you probably know that you can climb up no higher than the level of the crown. But what most do not realize is that there’s a reason why it was rendered off limits. Evidently, there seems to be a secret room in the torch, offering spectacular views of the city. Back in the day, a pier standing between Jersey City and Black Tom Island was blown up by German agents. The explosion that followed quickly ripped through the nearby buildings, dailymail.co.uk details. As a result, the debris ricocheted across the entire city as some hit the statue’s arm. Eventually, it made the secret route to the torch unsafe for visitors.

3 Hidden Rooms And Passageways Under Britannia Manor

Previously owned by the video game designer, Richard Garriott, Britannia Manor is a unique house and a huge throwback to the Medieval times. As impressive as it is, the house featured all sorts of bizarre things, from a human skeleton to dinosaur fossils and secret passageways, businessinsider.com shares. Another super weird item that Richard Garriott kept in the house was a 16th-century vampire hunting kit. Eventually, the game designer sold the house to purchase a much pricier and lavish residence. Well, the other one was piled high with too many secrets and strange collections, so maybe Richard wanted to get rid of them as well.

2 The Underground Railroad Beneath The Warren-Beck House

The Warren-Beck house is a historic house casually sandwiched between Faculty Club and Barker Center. This bizarre structure with an incongruous yellow façade was initially built for Professor Charles Beck in 1833. Charles Beck was a respected figure, a supporter of the anti-slavery movement. The house connected to the railroad thanks to a trap door, leading to a secret room. The yellow farmhouse lacks the Faculty Club’s sophistication as there’s also a small placard placed on the door, probably to inform you where you are. This little old house was later sold to Clarke Warren, thecrimson.com details, but upon his death, he bequeathed it to Harvard.

1 The Greek Project At The Greenbrier Luxury Resort

Dating back to the 1770s, the Greenbrier is, indeed, one of the most beautiful and lavish resorts in the world. The current building, located in West Virginia, was built in 1913, and only a few years later, the U.S government and the resort agreed to build a secret bunker right underneath the luxury resort, www. businessinsider.com shares. The facility was created for members of the Congress only so that they could hide there in the event of a war. This otherwise small bunker features a hospital, dormitory, a kitchen as well as an actual broadcast center. Eventually, the secret bunker was decommissioned and is now a major tourist attraction in the area.

References: thecrimson.com, dailymail.co.uk, atlasobscura.com