Sunless Sea
The Sunless Sea is a flooded cave biome in the upper cave band. It is a still body of water in a large hall, reached by descending through the deepslate rather than by any surface opening.
Finding it
The sea occupies the upper cave band, y111 to y221. Its water surface stands at y169 and is flat across the whole hall. The floor sits around y136 and rises into islands and beaches at the hall's edges.
What is there
Water to y169, with air above it up to the roof. The bed is deeperslate and black gravel. Dripstone hangs from the roof.
An Obsidian Rift that crosses the sea cuts a flooded trench through the bed. The trench fills to the sea's own level, so the surface is unbroken and the crack is entered by diving.
Mechanism
The biome is claimed by the partition when the band_upper noise exceeds SEA_CLAIM (0.33), inside the band BAND_UPPER. Its cavern is carved by spork:sea_cavern behind spork:sea_gate, a lazy-min wrapper bounded to y111..221.
Water is not aquifer noise. NoiseBasedAquiferMixin gives any aquifer cell in or within 16 blocks of the sea's claim a fluid status of water to WorldFrame.SUNLESS_SEA_LEVEL (y170), which is why the surface is flat and why neighbouring dry caves are walled off rather than draining into it.
Bed height comes from SEA_FLOOR_Y (BAND_UPPER[0] + 25) with a linear gradient of SEA_FLOOR_K density per block over SEA_FLOOR_SPAN.