Tunnel Network

A Tunnel Network is any collection of buildings that link to a large underground catacomb of tunnels that allow for stealthy travel around a locale. Sometimes these might have been built by dedicated criminal networks for the sake of transporting things covertly from place to place. Sometimes they're built during times of war to allow a way to sneak past, or sneak up on, enemies. Less common uses of Tunnel Networks include avoiding a dangerous climate aboveground, or to serve as the equivalent of roads for those who live Beneath the Earth. Yet another use would be that shifty government laboratory that is not only underground but randomly dusty.

In the competitive gaming world, this means cheap near instant transport between any two points. A shrewd gamer will be able to take advantage of this and employ what is called tunnel popping, quickly transferring units in and out of tunnels, and all around the map to a devastating effect.

In other media, while not allowing for something as cheap as delivering a Zerg Rush to your doorstep, can allow for secretive travel between locations hiding both literal and logistical foot prints.

For its high tech equivalent, see Portal Network. Occasionally related to Absurdly Spacious Sewer. A Secret Underground Passage sometimes leads to this, but is technically a different trope. This refers specifically to a massive underground network of tunnels, not a single tunnel used for a singular purpose.

Examples of Tunnel Network include:

Film

The Geonosians in Star Wars Clone Wars.
The Groundhog in Caddyshack has something like this.
The Great Escape's first phase involves building one of these.
In Real Genius, steam tunnels are how Lazlo gets around Pacific Tech unseen—they are modeled on the very real tunnels beneath Caltech.

Literature

In Robert Heinlein's Starship Troopers, Bug colonies had vast underground tunnel networks that allowed them to pop up and attack the MI on the surface of the planet.
In Good Omens, there's a sequence where various New Age beliefs start coming true, of which one is the conspiracy theory that the Secret Masters of the World live beneath the Earth and travel around through an extensive tunnel system. This results in things like gardening programs offering tips on what to do if a Secret Master of the World pops up in the middle of your flower bed.
In Pyrates, there's a large network of tunnels and caves underneath New York City, used both by homeless people simply to survive, and by smugglers and thieves.
The Ice Tunnels below the Castle in Septimus Heap.
In Warrior Cats, the interconnected rabbit warrens underneath WindClan's forest territory, and the natural cave system underneath ThunderClan and WindClan's territories by the lake.
Dresden Files depicts a semi-fictional Chicago "Undertown".

Live-Action TV

On Hogan's Heroes they had a network of tunnels under the camp, and leading out of camp.
2000's Secret Agent Man (no, not the one with Patrick McGoohan) had an underground highway system that allowed the agents to drive great distances at high speed out of sight of anyone on the surface.
The Get Smart Headquarters has this.
The TV Beauty and The Beast had an extensive underground network in association with the NY subway system, with a whole Secret Society down there.

Tabletop Games

Traveller Classic Double Adventure Death Station. The drugged humans on a space station have cut through the deck and cut holes in the fuel tanks to create underdeck passages throughout the ship.
Dungeons & Dragons. The Drow can move along underground passageways and emerge at various points on the surface.
Call of Cthulhu (tabletop game). Several adventures with ghouls and chthonians give them underground tunnel networks. The ghouls' tunnels often connect to graveyards.

Video Games

The Zerg in StarCraft have Nydus Canals.
The GLA of Command & Conquer: Generals have, well (hey, guess what?), Tunnel Networks.
Dawn Of War's Imperial Guard buildings have this capacity, as well as certain catacombs in the Necron stronghold mission.
Elaborate tunnel networks are typically created by many players of Minecraft.
In the The Lord of the Rings: The Battle For Middle-Earth 2 game, both the Dwarven faction and the Goblin faction had resource-mining structures that doubled as entrances to their respective Tunnel Networks.
Since fortresses in Dwarf Fortress tend to be mainly underground anyway, they usually incorporate some degree of this. Dwarven and goblin civilisations also create large tunnels linking settlements created during worldgen, and beneath those there are three levels of naturally-formed cavern system that blur the line somewhat between this trope and Beneath the Earth.
Advanced Strategic Command allows to pool resources in buildings via either pipelines constructed by bulldozers or inconvenient direct contact of buildings. Not only this saves bothering with transport, but producing some units require more materials than the factory or dock can hold. Buried pipelines are more expensive than open ones, but preferrable: they doesn't hinder unit movement and to break one the enemy must bring a construction unit to it instead of simply bombing. Construction ships can build underwater pipelines to connect islands.
Fallout 3 has a large number of subway stations that work as a limited Tunnel Network, given that much of the subway system has already collapsed.
Dragon Age has the Deep Roads, formerly a highway system for the dwarven kingdoms but now almost entirely held by the darkspawn.
Tunnels under Rome serve as fast travel systems in Assassin's Creed: Brotherhood.
Age of Mythology had a god power that created a tunnel that you could put in two different spots to transport units via the two tunnels.
The sewers (and abandoned sewers) under Paragon City in City of Heroes, and the abandoned subway system under Praetoria.

Web Comics

Annyseed features a large underground tunnel network that connects many of the residents of Skull Valley. In the webcomic we see Anny and Winston use one of the tunnels to get from Professor Tripadiculous' secret lab to Hamish the Swamp Dragon's cave.
The eponymous institution in Tales Of Gnosis College has an extensive network of steam tunnels running underneath its campus, which it happens to share with an adjacent Catholic women's college. Eventually some enterprising students put them to the obvious use.

Western Animation

In Pound Puppies (2010), the dogs base their operations in a central underground hub and use a series of tunnels beneath the shelter grounds to get themselves and puppies into and out of the pound.

Real Life

Spider Holes in Vietnam.
Catacombs.
Along the U.S./Mexican border alien smugglers sometimes dig tunnels to allow their customers to pass under the border fence.
North Korea has dug incursion tunnels under the DMZ for use during an invasion of South Korea.
Japan and Germany both had tunnel networks during WWII. Especially Japan.
Palestinians have tunnels in Gaza and the West Bank that are used to, among other things, facilitate trade
Iron Mountain in western Pennsylvania, where the U.S. government is prepared to hide in case of enemy attack.
Prairie dogs dig vast networks of tunnels.
Portland, Oregon, has a nifty system of tunnels, popularly known as the Shanghai tunnels, which according to possibly dubious 19th century historical record, were used to kidnap men (known as "shanghaiing") as free labor on the ships that sailed in and out of the city's port.
More mundanely, many cities have tunnel networks with trains running though them.
- Toronto and Montreal have networks of pedestrian tunnels connecting office towers, stores, and mass-transit hubs in their core downtown neighbourhoods. Toronto's PATH and Montreal's RÉSO each have approximately 20 miles of tunnels.
Fortresses of the Maginot Line were connected by tunnels. Pity they proved to be so useless.
The city of Tabor in Hussite War-era Bohemia had a full tunnel system dug beneath it (this was the early 15th century!). The system was designed so that the militia could rush through the tunnels to the heart of town if an enemy attacked, while the enemy was forced to negotiate a series of narrow streets so that no matter where they emerged, they'd walk into a massed barrage. The tunnels were full of food and beer to sustain the militia in case of siege as well.
Walt Disney World has such a network to hide certain aspects of park operations from the customers (trash collection being primary).