--Konoha Underground--
Beneath Konoha is an intricate system of tunnels, passages, and whatnot that are a combination of the village's sewer system and the various secret passages built by either the village itself or individual families within the village. These are thanks to the firm, dry earth beneath Konohagakure, a necessity to support the growth of the many trees around the village.
Officially, any villagers know about this underground system, but few non-ninjas know that it consists of more than the sewer system. Shinobis and kunoichis know about the secret passages from their years at the academy, where a few passages are designated as emergency escape routes and emergency drills lead the students down through the passages. However, for even most ninjas, that is all the experience they have of the passages, their knowledge limited to only those two or three that they were led down through when young. Then, there are those privileged enough to be members of prominent/large clans (ex: Hyuuga, Uchiha, Inuzuka) who have knowledge of their own secret passages--although, even these families don't necessarily know of all the passages leading into their homes--to use in times of trouble. The group to officially hold the most extensive knowledge is, of course, the elite that serve as ANBU or Oinin and the hokage. They have access to detailed and accurate maps of the entire system.
Unofficially, though, there are others who could be just as knowledgeable: the people who are especially cunning/intelligent and make constant use of the passages for whatever reason, whether it be for good or bad or just completely random. The frequency and purpose of their use determines just how much of the Konoha Underground they know about. Some even build secret hideouts (ex: Orochimaru's lab) within the maze of passages.
The Konoha Underground can lead to just about anywhere in the village as well as out. The difficulty is figuring out how to get to the intended destination; it is quite possible to be hopelessly lost in the maze of passages. All the tunnels that lead out of the village are carefully guarded by ANBU and rigged with alarms, so attempting to enter/leave from below is just as difficult as attempting to enter from above if not more so as any unauthorized entry/exit will be treated as an immediate threat.
