Secret Underground Passage

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, and are sometimes built into houses. 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.

A staple of Dungeon Towns. Compare Absurdly Spacious Sewer. Subtrope of Secret Path. Frequently has a Bookcase Passage as one end if it goes into a house or other building.

Examples of Secret Underground Passage include:

Anime & Manga

There's one in Detective Academy 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 with 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.
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.

Board Games

Clue provides two as quick ways to access the opposite corners of the board.

Comic Books

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.

Film - Animated

In Madagascar, the penguins are introduced digging a tunnel to escape from the zoo. In the Spin-Off The Penguins of Madagascar, it's revealed that they have a network of 15 escape tunnels.

Film - Live Action

National Treasure 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.

Literature

In Nightrunner, there's one in the fourth book leading from the workshop out to a barn.
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 Baby Sitters 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's 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.
There are at least two explicitly underground passages in the Harry Potter books—the tunnel between Hogwarts and Honeyduke's; and the tunnel between the Whomping Willow and the Shrieking Shack. There are numerous other tunnels and passages in Hogwarts, though, any of which might include an "underground" component.

Live Action TV

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?
On Leverage this occurs during the 10 Lil' Grifters Job. It is how the killer gets away and frames Nate.

Video Games

In Dragon Age, there's one between the windmill and Redcliffe Castle.
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.

Western Animation

Avatar The Last Airbender's secret tunnel became a meme.
Being a popular mystery trope, this happens in many episodes of Scooby Doo.

Real Life

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.