Tunnels are broadly divided into two categories: Tunnels to something, and Tunnels from something. There are further subdivision which also include: tunnels you go into, but don’t come out; and Tunnels you come out, but don’t go into.
A tunnel is typically defined as any enclosed space with two exits, which may, at any moment, collapse, trap and kill everyone inside. They are the engineering opposite of a bridge, which is broadly defined as any precarious platform suspended between 2 peaks, which may, at any moment, collapse, free-fall and kill everyone on it.
It is said that Jesus' followers invented the tunnel, when they created a self-supporting hole from the outside of a tomb, to the inside of the tomb, where they replaced their leader’s cadaver with a living body-double. From this point on, people realized that tunnels were an effective way to allow matter to pass through other matter, without the laws of physics objecting too violently or terminally.
However, prominent figures suggest the tunnels have existed long before Jesus, particularly in the form of the donut. However stiff opposition states that the donut does not demonstrate a tunnel, but in fact, demonstrates a multi-directional hole.
Underwater tunnels add another dimension to the traditional tunnel concept. Their construction is trickier by far, and usually circumvented through the use of boats and planes. However, one famous underwater tunnel, despite its existence, continues to be ignored by a small and unintelligible group of swimmers who believe that freezing cold temperatures and a traveling time of 20 hours is preferable to 30 minutes in a car.
Underwater tunnels, when they inevitable collapse, offer no chance of survival to those inside, unlike traditional tunnels – where those trapped are offered a very slim chance of Sting coming to the rescue.
Tunnels you go into, but don’t come out
Often simply referred to as a ‘Tun’. Any given Tunnel may become a Tun under the appropriate conditions. Typically, if the glue and sticky tape fails, or if the construction workers decided it would be easier to work around said object than dig underneath, a tunnel will inevitably become a Tun. At that point, pray that you were like the construction workers, and decided to take the long way, instead of trying to avoid a toll booth somewhere.
Sometimes a Tun may form when a Tunnel appears structurally sound, though this is rare. See Princess Diana.
Tunnels you come out of, but don’t go into.
The most notable example is the Penis. Everyone alive has come out of a tunnel they didn’t deliberately go into, when they were born. However, at a later date, they may have returned, but not to the same one from which they came. Hopefully.