Strange Secret Entrance

A Strange Secret Entrance is the hidden place that you access by seemingly impossible means. When Alice first tells Bob about the Strange Secret Entrance and how it is reached, Bob (usually) doesn't believe what he's hearing until he sees Alice easily access this hidden area with the exact same method that she described to Bob. Bonus Points if Bob can't make his way in at first, even by copying Alice exactly.

May involve passing through a Cool Gate. In video games, frequently overlaps with Developer's Room.

Film

 * In the first Oh, God! movie, God meets the main character on a floor that doesn't exist. It's something like floor 49 on a building that goes up to the 30th floor.

Literature

 * In the Harry Potter series is Platform Nine-and-Three-Quarters, a secret train platform between numbers 9 and 10, at the King's Cross railway station in London. Wizards and witches (but not Muggles) can get to this hidden stop by walking through a certain brick pillar that separates Platform 9 from Platform 10.
 * Harry Potter also has Diagon Alley, whose most mundane entrance can be accessed by going through an inn (which seems to be enchanted to keep Muggles from seeing it anyway) and tapping exactly the right spot on a brick wall with a wand.
 * A third Harry Potter example is the Room of Requirement in Hogwarts, which one accesses by walking past its door while consciously thinking of needing something. The room then supplies what you need.
 * There are two ways to enter the Ministry of Magic: one is an elevator disguised as a phone booth, the other is flushing yourself down a public toilet.
 * The eponymous location in The Secret of Platform 13 gives a good example.
 * Narnia is, at least in the The Lion, the Witch and the Wardrobe, accessed by walking through the wardrobe.
 * Other ways to get in include trans-dimensionally-travelling magic rings, accidentally falling through a 2D painting and
 * The Wheel of Time: The Eye of The World can only be found once by any person, with a single exception. It moves, but always within a specific, very dangerous region.
 * Similarly are the worlds of the Finn (Snake and Fox People) which are accessed through two doorways with eye-wrenching curvature.  The main characters have an argument when three of them come out of the door and are informed that at most one person should be inside and that one of them was using magic, which is a no-no.
 * Wayside School is thirty stories tall, but of those thirty, they forgot to build the nineteenth. It does not exist, and its resident teacher Miss Zarves and her entire class are entirely imaginary. Nonetheless, a student ends up there by accident in one story.

Live Action TV

 * In Torchwood, the Hub could be entered by a lift next to the fountain in Roald Dahl Plass that was concealed by a perception filter. There was also a more mundane secret entrance in a tourism office.

Video Games

 * Some of the hidden levels in the Doom series were like this. For example, getting to one secret level required you to blow yourself off a ledge by firing your rocket launcher into a wall at point blank range.
 * Some Rocket Jump, sticky jump, dispenser jump, etc. locations in Team Fortress 2.
 * The 3D Grand Theft Auto games have at least one inaccessible area per game with a "You're not supposed to be able to get here" sign - areas which are still reachable, through very obtuse means.
 * San Andreas lampshades this in a way, by having a sign that reads "There are no Easter eggs here, go away" on top of a massive bridge, and in itself features a presumably unintentional version of this with the legendary "hidden interiors".
 * The aptly-named "Pyramid of the Forbidden" in Commander Keen episode 4, reached by going into the basement of the "Pyramid of the Moons" and coercing twelve inch-worms to come together, at which point they form a giant foot which transports Keen.
 * Duke Nukem 3D had some areas like this. One level had an area with the message "You're not supposed to be here" and later had "The Dopefish lives!" at the bottom of a pillar (though both are reachable without cheating by collecting the jetpack in the previous level), and an earlier level had a hidden area asking "How did you get here?".
 * Some secret levels in Donkey Kong Country are accessed by unintuitively jumping down "bottomless" pits into off-screen barrel cannons.

Western Animation

 * There's a village in Generator Rex that is hidden behind some sort of invisibility field. You can only get into it if you know where to walk into/through it.
 * In the Wayside School series, there is an imaginary 19th floor that is visited by at least one character despite its lack of existence.

Real Life

 * Many real world Rapid Transit systems (usually the older ones) are sprinkled with abandoned platforms,stations or even entire lines. They often must be reached through nondescript doors, hatches or even by walking through the tunnels themselves. Often these locations will be stuck in some sort of retro stasis reflecting the time period in which they were closed.
 * The New York City Subway has more disused and abandoned stations perhaps than any other. Chief among these is the City Hall Station which despite being the showcase station for the entire IRT, was closed in 1945 due to its 5-car loop configuration and then later hidden from public view due to post 9/11 terrorism concerns. Can be accessed by riding out of service 6 trains or by running through the tunnels.
 * 76th St station was rumored to have been constructed on the Fulton St line as a terminal for the C, but later closed up and abandoned with the NYCTA steadfastly denying its existence ever since.
 * The Toronto TTC has the infamous Lower Bay station that was closed due to changed service patterns. It shows up as a filming location in many movies and television shows.
 * PATH has the quirky Platform H at Newark Penn Station. This platform is located on a level above all the other main railroad tracks and only used by inbound PATH trains discharging passengers making connections at Newark Penn. Because Platform H is discharge only, most people, despite using it, have no idea how to get to it as they are always coming from it and there is no signage directing people there.
 * London Tube has several good real life examples, some complete with WWII posters from the time they were used as bomb raid shelters. Well-documented website exists.