->''Not more than one secret room or passage is allowable, and such a passage may only be in a house or building for which it is appropriate by age or purpose.''
-->'''Father Ronald Knox''', "[[FairPlayWhodunnit Ten Commandments]]"

Often discovered by a snooping hero, the SecretUndergroundPassage is a popular trope in mystery and adventure-themed works. They are often a secret way of going from point A to point B without being detected, and are sometimes built into houses. If bad guys built it, then the SecretUndergroundPassage is often used as a place to smuggle things and stash loot, or hide prisoners. If good guys built it, it's often a place to escape persecution, sneak past danger, or hide something important they don't want the bad guys to find.

This is sometimes discovered when the hero notices strange sounds - often human voices - and tries to find their source. Other times, it's discovered simply by accident when poking around, or even just [[ShaggySearchTechnique leaning on the correct bookshelf and conveniently revealing the hidden passageway]].

Sometimes the SecretUndergroundPassage is not just a single passage, but links to a [[TunnelNetwork huge network of tunnels under the ground]].

A staple of {{Dungeon Town}}s. Compare AbsurdlySpaciousSewer. Subtrope of SecretPassage.
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!!Examples:

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[[folder:Anime & Manga]]
* There's one in ''DetectiveAcademyQ'', where they found out all this while that their school building has a secret passage. Once there, they find what seems to be evidence of a person imprisoned underneath, with a diary written in his own blood.
* There are at least two in ''LightNovel/{{Corsair}}'', one beneath Sesaam's bed that allows Canale to escape after murdering him and another in one of the rooms of [[spoiler:the D'Aubigne mansion, which Canale uses to rescue Aura.]]
* In Ubel Blatt, Koizell and Ato use an ancient subterranean passage to cross under the mountains so as to avoid Glenn's armies, but Glenn suspected that the notorious "Hero Killer" might possibly know of the secret tunnel although it seemed very unlikely.
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[[folder:Board Games]]
* ''TabletopGame/{{Clue}}'' provides two as quick ways to access the opposite corners of the board. It also prevented a single unscrupulous player from blocking you inside a room by standing right in front of the only door out of those corner rooms.
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[[folder:Comic Books]]
* The tunnel connecting the garage in the [[Franchise/{{Batman}} Bat-Cave]] to the roads into Gotham in the various ''Franchise/{{Batman}}'' incarnations. Whether this is a TunnelNetwork [[DependingOnTheWriter depends on the writer]] as always.
* ''SteelgripStarkeyAndTheAllPurposePowerTool'' has an underground tunnel near the New York Public library leading to the Star Key Enterprises headquarters.
* Von Goosewing made one into the castle of WesternAnimation/CountDuckula. Of course, it wasn't a very ''secret'' passage - Igor and Duckula figured out what he was up to long before he finished, and occasionally entertained themselves by watching him dig.
* In ''ComicBook/PrinceOfPersiaTheGraphicNovel'', there are secret passages connecting all the wells, new and old, known only to the Guardians of the Waters.
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[[folder:Film - Animated]]
* In ''WesternAnimation/{{Madagascar}}'', the penguins are introduced digging a tunnel to escape from the zoo. In the SpinOff ''WesternAnimation/ThePenguinsOfMadagascar'', it's revealed that they have a network of 15 escape tunnels.
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[[folder:Film - Live Action]]
* ''Film/NationalTreasure 2: The Book of Secrets'' has the main character take the president through a secret underground passage in search of answers to a historical mystery.
* ''Film/SnowWhiteAndTheHuntsman''. Snow White and her knights in shining armor are riding on the Queen's castle, which is on an island and so can only be attacked in the short time the tide is out. Cut to the dwarves sneaking in through the sewers.
-->"We're promised gold and what do we get? Poo!"
-->"Eww! I think that's one of mine."

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[[folder:Literature]]
* ''Literature/TheRadix'' has two: one in Prince Zaki's residence, another in Carl Jung's castle.
* In ''Nightrunner'', there's one in the fourth book leading from the workshop out to a barn.
* In ''Literature/FeliksNetAndNika'' entire sewer system of Warsaw becomes one when Golem Golem escapes into it. Those are [[AbsurdlySpaciousSewer Absurdly Spacious Sewers]] and many of them run next to basements. Golem Golem, being [[SuperStrength massive robot]], just punches through whenever he wants to enter someplace.
* ''{{Pyrates}}'' is made of this. There's a secret underground path beneath the family home, which has been in existence for centuries. A great deal of the book takes place underneath New York City, and the underground paths lead to several places including the bad guys' hideout, and different parts of NYC.
* ''The Ghost at Dawn's House'' in ''TheBabysittersClub'' series. Her house was built with a secret tunnel (with a secret panel in her bedroom wall) to assist people on the underground railway.
* There are a few in Alexandre Dumas's ''Literature/TheThreeMusketeers'' series. In a footnote to the Oxford World's Classics edition of ''The Vicomte De Bragelonne]]'', editor David Coward remarks on the historical accuracy of one such passage:
-->There was a communicating tunnel in Fouquet's town house, but not at Saint-Mandé. [[RuleOfCool But Dumas was not a man to waste a good subterranean passage.]]
* In the last chapter of ''The Extraordinary Adventures of Literature/ArseneLupin, [[GentlemanThief Gentleman-Burglar]]'', Lupin discovers a hidden underground tunnel between a house and a nearby church, and uses it to abscond with the furnishings. Secret underground passages also occur in other Lupin stories.
* ''Literature/TheAdventuresOfArchieReynolds'' has one used by smugglers, that the heroes stumble across while exploring the neighborhood.
* ''Literature/{{Beowulf}}'' has a secret underground passage into the dragon's lair; a thief uses it to steal a goblet from the dragon's hoard while the dragon is sleeping, causing the dragon to go on a rampage when it figures out that it's missing.
* ''Literature/TheHobbit'' has a secret underground passage into Smaug's lair; Bilbo uses it to steal a goblet from Smaug's hoard while Smaug is sleeping, causing Smaug to go on a rampage when he figures out that it's missing.
* A popular stock trope of British children's mystery/adventure author EnidBlyton.
* In the ''Literature/WarriorCats'' series, there's a culvert under the road in [=ShadowClan=] territory that is considered by the cats to be a secret passage; it can be used for safely crossing the road and for sneaking onto the other Clan's territory. [=ThunderClan=] didn't know about it for ''years'' until Fireheart and Sandstorm discovered it.
* In Creator/VernorVinge's novella, "Literature/TrueNames", the castle in the Other Plane where the Coven meet has only one well-guarded way to enter, but, as Erythrina reveals, it has secret passages that can be used to ''leave''. This being virtual reality, making them one-way is not difficult.
* In AndreNorton's ''Literature/IceCrown'', the gates of the queen's city are closed. Nelis, however, knows of one of these.
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[[folder:Live Action TV]]
* In the ''Series/DoctorWho'' serial "Genesis of the Daleks" there is a secret underground tunnel between the Kaled and Thal cities that Davros uses to sneak the newly created Daleks into the Thal city. This leads to a bit of FridgeLogic. If the Kaleds knew there was a tunnel between the two cities, why didn't they use it to smuggle normal troops into the Thal city and end the war years ago?
* On Leverage this occurs during the 10 Lil' Grifters Job. [[spoiler:It is how the killer gets away and frames Nate.]]
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[[folder:Pinball]]
* In Creator/WilliamsElectronics' ''Pinball/WhiteWater'', there's a small and barely-visible passage near the top of the table, under the upper playfield. It's usually only accessible via a soft shot up the left orbit, and making it automatically advances the player's raft.
-->'''Willie:''' "You found the Secret Passage!"
* The ''Pinball/PinkPanther'' pinball has a Hidden Staircase and a Secret Passage; they are used to lock balls for multiball.
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[[folder:VideoGames]]
* In ''VideoGame/DragonAgeOrigins'', there's one between the windmill and Redcliffe Castle.
* There's a number of these in the ''Zelda'' series, especially ''VideoGame/TheLegendOfZelda''.
* In ''VideoGame/CallOfCthulhuDarkCornersOfTheEarth'', an old smugglers' tunnel is the route through which the army attempts to get into an otherwise inaccessible building. [[spoiler:All the soldiers die, and the player is the only one left to go through them.]]
* Underground train tunnels and subways, sewers, and bunkers in the ''VideoGame/{{Fallout}}'' series.
* The secret underground tunnels in ''VisualNovel/UminekoNoNakuKoroNi'' were, in the spirit of Knox's 5th, historically and plausibly justified. These are never involved in any of the closed door mysteries.
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[[folder:WesternAnimation]]
* The secret tunnel in ''WesternAnimation/AvatarTheLastAirbender'' became a meme.
* Being a popular mystery trope, this happens in many episodes of ''Franchise/ScoobyDoo''.

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[[folder:RealLife]]
* Some cities in the US that once hosted large numbers of the Klu Klux Klan will have one or more of these linking common meeting places. The city of Cañon City, Colorado has one linking a old bar's basement and another local business. Needless to say, the shameful origin of the passages often precludes them being tourist attractions.
* There are secret tunnels beneath the Canadian city of Moose Jaw, used to house Chinese immigrants working in the city laundries, as well as rumrunning during prohibition.
** Though many of said tunnels were actually mundane things like coal storage bins.
* Secret tunntels were built by east Berliners under the Berlin Wall in various escape attempts.
* Extensive networks of tunnels were dug underneath Portland, Oregon and used for highly unsavory activities. (This is a major understatement.)Historical societies have been working to excavate them and to document stories of people who were victims of the human trafficking which they were used for.


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