Often discovered by a snooping hero, the Secret Underground Passage 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, usually built into houses or castles. If bad guys built it, then the Secret Underground Passage 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 leaning on the correct bookshelf and conveniently revealing the hidden passageway.
Sometimes the Secret Underground Passage is not just a single passage, but links to a huge network of tunnels under the ground.
- 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.
- There's one in Detective School Q, 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 Corsair, one beneath Sesaam's bed that allows Canale to escape after murdering him and another in one of the rooms of the D'Aubigne mansion, which Canale uses to rescue Aura.
- 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.
- The tunnel connecting the garage in the Bat-Cave to the roads into Gotham in the various Batman incarnations. Whether this is a Tunnel Network depends on the writer as always.
- Steelgrip Starkey and the All-Purpose Power Tool 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 Count Duckula. 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 Prince of Persia: The Graphic Novel, there are secret passages connecting all the wells, new and old, known only to the Guardians of the Waters.
- One is dug over several issues of Revival by the militia to undermine the quarantine.
- National Treasure 2: Book of Secrets has the main character take the president through a secret underground passage in search of answers to a historical mystery.
- Snow White and the Huntsman. 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."
- The house in Clue has the same two as in the board game. Both are used in murders: to escape the kitchen after killing the cook, and to enter the lounge to kill the motorist.
- Involved in one of the It Was Here, I Swear! gags in Abbott and Costello Meet Frankenstein.
- In Avengers: Age of Ultron, Tony finds one of these when investigating Baron von Strucker's lair. Judging by his attitude after discovering it, he was anxiously expecting one.
- In Harry Potter, Much of Hogwarts castle is built with several secret passages that the students use to get around school quickly. The castle has so much and some are so well hidden few people know the exact locations of each one. The only three plot significant secret passages used in the books are passages leading into Hogsmeade Village:
- One leading to the cellar of Honeydukes candy store.
- Underneath The Whomping Willow into the shrieking shack.
- The portrait of Ariana Dumbledore in Aberforth's home leading to the Room of Requirement. This last didn't exist until Deathly Hallows, and was created by the Room of Requirement.
- The Radix 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.
- The Castle Canyons are secret passages in the Castle of Septimus Heap. They are used by Jenna, Septimus and Marcellus Pye in Darke to get around while the Castle is swamped by a Darke Domaine.
- In Feliks, Net & Nika entire sewer system of Warsaw becomes one when Golem Golem escapes into it. Those are Absurdly Spacious Sewers and many of them run next to basements. Golem Golem, being 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 The Babysitters Club 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' The Three Musketeers 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. But Dumas was not a man to waste a good subterranean passage.
- In the last chapter of The Extraordinary Adventures of Arsène Lupin, 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.
- The Adventures of Archie Reynolds has one used by smugglers, that the heroes stumble across while exploring the neighborhood.
- 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.
- The Hobbit 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 Enid Blyton.
- In the Warrior Cats 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 Vernor Vinge's novella, "True Names", 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 Andre Norton's Ice Crown, the gates of the queen's city are closed. Nelis, however, knows of one of these.
- The Executioner.
- In "Stony Man Doctrine" the terrorist Psycho for Hire assassinates his KGB handler, who lives in a guarded compound, by entering via the one place not covered by guards and security sensors — the secret escape tunnel. If guards are posted on the tunnel it could lead to Bodyguard Betrayal. Likewise an alarm sensor would eventually be activated by rats or rainwater, again giving away the location. The engineer who designed the tunnel was transferred to another part of the world, but the terrorist tracked him down and bribed him with gold to start a new life in the West.
- In "Canadian Crisis", Bolan discovers a network of passages and hidden doors built into the walls of a Montreal hotel. He uses these to wage guerilla warfare on a Mafia conference being held there, and eventually finds out that the passages were put in during construction by Quebec nationalists planning a terrorist attack on dignitaries who would be staying in the hotel during the upcoming 1976 Olympics.
- In The Girl from the Miracles District, there are two:
- There's a tunnel beneath the street connecting burlesque theatre Music Box with bordello Terra del Fuego. The two establishments are owned by twins, and because "private shows" with the girls are not allowed in the Music Box, the clients who would like them but can't or shouldn't be seen entering Terra del Fuego use this way.
- There's another secret passage between Order HQ and the house of its leader, Irena, put in place so that no-one could track down where she lives by stalking her, and also so that she could escape quickly if need be.
- Friday. Friday stays with some friends at their Big Fancy House, which has a secret room plus escape tunnel in case they're raided by the authorities. Later when she's on the run and realises she's close to their house, Friday has to work out where the passage is (and evade its defenses) from what they told her.
- Watership Down. The 'slack run' was dug by the rabbits as a means of coming up behind some enemy and attacking them. When the warren is destroyed by men plugging the runs and filling the warren with Deadly Gas, a couple of rabbits use the slack run to escape.
- In the Doctor Who 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 Fridge Logic. 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?
- However when trying to get back to the city Sarah and Harry are attacked by Mutants, showing that it is still a dangerous trek to get to the tunnels and that it would be difficult to get an army there.
- On Leverage, this occurs during the 10 Lil' Grifters Job. It is how the killer gets away and frames Nate. It's also where the blueprints are hidden.
- Game of Thrones. In Season 7, Tyrion Lannister sends an army to besiege Casterly Rock where he grew up. Normally this would be a costly affair, but it turns out while his father trained the Lannister army and built the castle's defences, he put his least favourite son in charge of the sewers, where Tyrion built a secret passage so he could smuggle prostitutes in without his father noticing. A small force enters the castle and opens the gates from the inside.
- The Avengers had some fun with this. Emma Peel in a fetish outfit is being whipped by the Villain of the Week, but his whip catches on a torch pulling it out of the wall, and activating the requisite trapdoor which he falls into.
- In Williams Electronics' White Water, 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 Pink Panther pinball has a Hidden Staircase and a Secret Passage; they are used to lock balls for multiball.
- In Dragon Age: Origins, there's one between the windmill and Redcliffe Castle, used by the generations of Arls of Redcliffe as an escape route for their families in case of a siege. In the game, the Player Character uses it to sneak into the castle, however. Later on, in Dragon Age: Inquisition, the Redcliffe Castle is taken over by a Tevinter Magister, so (if you decide to pursue this story branch), Leliana—a former companion of Origins' PC—smuggles a cadre of her agents inside via the same underground passage.
- There's a number of these in the Zelda series, especially The Legend of Zelda.
- In Call of Cthulhu: Dark Corners of the Earth, an old smugglers' tunnel is the route through which the army attempts to get into an otherwise inaccessible building. 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 Fallout series.
- The secret underground tunnels in Umineko: When They Cry were, in the spirit of Knox's 5th, historically and plausibly justified. These are never involved in any of the closed door mysteries.
- A staple of the Nancy Drew series.
- A variant of this is the basis for one of the side-quests in the Original Campaign of Neverwinter Nights. If you talk to a child standing outside a certain house, she tells you about a secret passage in her house which opens when a certain word is said. Overlaps with Open Sesame.
- In The Beast Legion Master Surya reveals an underground passage to the anointment room in Issue 5
- Keychain of Creation breaks down exactly why some of Tolkein's ideas on secret doors were actually pretty bad.
- Girl Genius
- Sturmhalten's deep down, beneath it's spacious sewers, is full of secret passages and used to be frequently traveled by official Sturmhalten sewer rats (explorers who kill loose monsters) but was closed by the Prince supposedly due to it becoming too dangerous though it was more likely to hide the fact that he was providing a large contingent of Geisterdamen a shelter and room for research down there.
- The "sneaky gate" in Mechanicsburg leads to a passage underneath the town which the Jägers use as a loophole to get to Mamma's before the Doom Bell rings since they technically haven't entered Mechanicsburg itself.
- Dice Funk: Rinaldo looks for a secret passage in every building. He has yet to find one.
- Some cities in the US that once hosted large numbers of the Ku 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 tunnels 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.
- Drug smugglers have dug a number of tunnels underneath the US/Mexican border.