|This article is about a location in Umineko no Naku Koro ni.|
The Underground Passage is a long tunnel underneath Rokkenjima.
The underground passage contains some dungeon cells filled with old cages and torture devices. One end leads to the basement of the hidden mansion Kuwadorian. The other end comes above ground through a grate behind the main mansion.
The dungeons are first shown in Alliance of the Golden Witch. Gaap uses her powers to transport the survivors of the first twilight all the way from the mansion's dining room to an underground cell. Kinzo later reveals to the group that these are connected to Kuwadorian, and Nanjo has walked through this passage before.
Requiem of the Golden Witch reveals that the passage and its dungeons are actually remnants of an old Japanese military base that existed on Rokkenjima during World War II. The two-kilometer long passage leads to a submarine dock hidden on the other side of the island. After Kinzo purchased the abandoned island, he built a room within the base to house his stockpile of gold. The passage is also where 900 tons of explosives are stored, yet more leftovers from the military base. They have enough power to completely destroy the mansion and are rigged to a clock inside the gold room that will go off at 24:00 if it is activated.