[Flavah Menu] Dungeon Raisons d'etre

Today's flavor menu is a quick little crib you can use to justify the existence of an underground complex in your campaign. When I'm writing an adventure, I often start with one of these to focus my dungeon design -- once you have the motive, it becomes much easier to plan the dungeon.

• Catacomb: The dungeon was built to house the remains of the followers of an ancient cult. The tunnels are haphazardly planned, because they were added to at need, rather than according to a grand design. Levels are often connected by narrow shafts or pits, because they were not built for frequent use. The ruins may contain chapels and other chambers where the Mysteries of the cult were performed to initiate new members.
• Refuge: The dungeon was built as a retreat for an entire village or community during times of war, or because the locals were the target of frequent raids from neighboring cultures or monsters. Because life has to go on, the dungeon is designed as an underground city-in-miniature, with residences, markets, storage areas, wells, bakeries, armories, and other specialized structures, both private and shared. Gaining entrance is often difficult, via a disguised or well-hidden portal, but once inside, navigation through the public areas is relatively easy. Some areas may resemble courtyards, open to the sky. The city of Petra in Jordan and the hidden city of Cappadocia in Turkey are classical examples.
• Mine: The dungeon was excavated to extract a valuable resource, either metal ore, a valuable gem, or the buried treasures of an older civilization. Each level may contain many galleries, that either follow a vein of ore in twisted fashion, or else are dug in a regular pattern, like a grid. Each level will most likely be connected by vertical shafts that are (or were) served by elevators or large baskets on a winch system. As lower levels are reached, water features and flooded areas become more common, and throughout is the possibility that the mine will breach natural features like limestone caves. The mine entrance will usually be easy to discover, because of the mounds of slag and other rubble stacked nearby.
• VIP tomb: The dungeon was built to house the body of an important person: a monarch, high priest, wizard, or hero. Repeated attempts by tomb robbers may have led to the more accessible parts being looted, but other areas remain intact, hidden by secret doors and the like. Monsters may be of the immortal guardian sort, like golems, along with strays that have moved in since. Lots of traps and false passages too.
• Prison: Like a VIP tomb, except designed to keep whatever's inside from getting out. Extremely difficult to enter, exit, or navigate, but time's heavy hand may have caused some of the defenses to crumble, making it easier to access. Whatever's in there was worth building a complex prison for that would last for centuries -- so tread lightly. Any monsters are either cell mates of the prisoner, or guardians set to prevent anyone from getting in or out. 
• Sunken City: An entire city that has since been buried underground, either by subsequent layers of occupation, or by a mudslide, sandstorm, or volcanic eruption. The old city may have been entirely cased in mud or ash, and subsequently hollowed out again by deliberate mining, occasional flooding, or the mindless burrowing of underground creatures. All is preserved as it was the moment the city was entombed, including its many treasures, which await those with enough patience or courage to recover them. Think Pompei or Herculaneum, or the Seattle Underground.