The Unremarkable Dungeon is located to the north-west of Terinyo. To find it, you must go to the Small Cave, and explore until you find some stairs going down. The only way to get to the High Mountain Village (and the guaranteed shop and companion there) is through the Unremarkable Dungeon (exit is at bottom level 8). The name "Unremarkable Dungeon" does not appear while playing the game. The dungeon is known only by its handle "UD"; however, if the player dies in the UD, the death report will specify "in an unremarkable dungeon".
Named in an ironic manner, the Unremarkable Dungeon is anything except unremarkable. It features the highest dungeon feature generation rate in ADOM, which means that at least one shop or level containing herbs is close to guaranteed. The same goes for pools, forges, altars and lesser vaults. Whether there is an increased rate of royal vaults is questionable. The Unremarkable Dungeon appears to also contain a much wider variety of features than the Puppy Cave or Village Dungeon; in particular, lesser vaults, forges, and possibly also surges of power are anomalously rare in the other early dungeons.
However, there are no truly guaranteed features in the UD. It is very possible to get a streak of entirely mundane levels. While there is arguably no increased chance to find a river, if they appear they may be more of an issue, since the route back through the Small Cave may well turn out to be virtual suicide.
Even a mundane level in the UD is remarkable for its moderately high danger level, considering it is located in the eastern, early game part of the Drakalor Chain, and the Small Cave is advertised as a good place for beginners. This means that starting the game in the SMC and descending through the UD is considered somewhat of a challenge, often rewarded with superior equipment (e.g. crystal tower shields, mithril scale mail) and very quick level and power increases and access to the HMV—provided the PC has what it takes.
There are no guaranteed NPCs in the Unremarkable Dungeon, just the random monsters found in any dungeon.