Tunnel

Tunnels were underground passage ways utilised by beings and mechanised equipment for access. They could be naturally occurring geological formations, (TV: Sanctuary) or they could be generated by by beings (like Silurians) at will or dug through physical or mechanised means. (TV: Doctor Who and the Silurians, Frontios)

On the planet Peladon the castle and the mountain surrounding it contained a honeycomb of tunnels, some seemingly constructed to lead into the castle while others were natural formations of the rock. (TV: The Curse of Peladon, The Monster of Peladon)

Likewise the Ice Warriors of Mars had an extensive tunnel network below the surface connecting their cities. (PROSE: GodEngine, The Dying Days)

On the planet Aridius there were tunnels and airlocks connecting one another. Whilst on this planet Ian Chesterton and the First Doctor lured and trapped a Dalek within one of these tunnels. (PROSE: The Chase)

Constructed tunnels
Tunnels were major engineering feats for civilisations to provide basic services like sewerage, water, cabling and mass transit. (PROSE: Reckless Engineering, TV: Attack of the Cybermen, Daleks in Manhattan, PROSE: Seeing I) In some cases the grandiose nature of large construction projects like tunnels leads them to be compared to other feats of construction. (PROSE: Molten Heart)

Tunnels utilised for water or sewage carriage or access often smelt of the substances they were being utilised for transport, unpleasantly so. (PROSE: Damaged Goods, The Fall of Yquatine)

In some cases tunnels did not lead anywhere they were dug to gain access for archaeological discovery or exploration. (PROSE: Doctor Who and the Dæmons) In locations such as the planet Menaxus the muddy surface meant tunnels could be dug utilising a water cannon. (PROSE: Theatre of Qar)

Railway tunnels
Underground railway tunnels were constructed often below cities for mass-transit networks, these tunnels were extremely long lived, often existing beyond city's lifespan. (TV: The Mysterious Planet (TV story) The Mysterious Planet, Orphan 55)

London, Novosibirsk and Paris were three cities on Earth that utilised underground tunnels for their railway network. (TV: The Web of Fear, City of Death, Orphan 55)

Other railway tunnels were constructed that allowed railway vehicles passage while remaining level such as from one side of a mound to another, or through another constructed area or locale. (TV: Day of the Daleks, Flatline, PROSE: Special Weapons)

The channel tunnel one Earth transversed a large body of water – the English Channel carrying railway vehicles. (PROSE: The Blue Angel)

Roadway tunnels
The Motorway on New Earth was a large sealed tunnel. (TV: Gridlock)

Other access tunnels
There were several tunnels below both the Thal and Kaled domes that exited into the wastelands. (TV: Genesis of the Daleks)

Maintenance access tunnels
To provide access for maintenance duties on research and other related bases maintenance tunnels were often located below the main working space of the base. (TV: The Satan Pit)

On spacecraft these were known as access tunnels. (PROSE: Return of the Living Dad, The Dimension Riders)

Mining
Tunnels were dug below the surface of a planet for the purposes of mining minerals. (TV: The Green Death), the disused tunnels were often repurposed. (PROSE: Parallel 59)

During the 22nd century Dalek invasion of Earth the Daleks tunnelled into the Earth's core, using physical slave labour without the use of machines. (TV: The Dalek Invasion of Earth)

Protection
During disasters and other natural or military-driven events tunnels, both constructed and natural were utilised by populations for protection from the elements and attacks. (PROSE: The Year of Intelligent Tigers)

Artificial tunnels
Artificial tunnels or non-permanent tunnels could be constructed from flexible plastics to allow safe access to areas or structures without exposure to exterior or hazardous environments. (PROSE: Eternity Weeps, The Monsters Inside)

An airlock tunnel was utilised by two spacecraft when docking to bridge the gaps between their respective docking ports/airlocks. (PROSE: Shakedown, Zamper) These were sometimes known as "boarding tubes". (PROSE: Original Sin)

A tunnel could also be constructed from normal space through to a craft moored in hyperspace. (PROSE: Original Sin)

Simulation tunnels
Some tunnels were created to simulate air flow, these were known as "wind tunnels" and utilised large movements of air to simulate wind movement. (PROSE: The Pit)