Obsidian Rift
An Obsidian Rift is a vertical crack that runs from a lava chamber in the upper cave band up through the deepslate to the caves of the stone leg. It is the legible way down.
Finding one
Rifts are rare and regional. They occur only inside a rift province, roughly a fifth of the map, and where they occur they stand a few hundred blocks apart. The crack is about 10 blocks across at its neck and widens to about 20 at its mouth.
What is there
Obsidian walls, veined with crying obsidian, from three blocks under the lava table and creeping up the crack. A lava table at y125 floors the chamber. The crack opens into the caves above at y373.
Where a rift crosses the Sunless Sea it is flooded to the sea's own level rather than open to the air, so the sea keeps its surface and the rift becomes a trench in the bed. Inside the sea's footprint the crack's lava table is given up to the water.
Mechanism
The rift is the zero-set of the slow rift_course noise inside the rift_province noise's high ground, above RIFT_PROVINCE_T (0.3). Half-widths are RIFT_W_MID 0.007, RIFT_W_TOP 0.014 and RIFT_W_BOT 0.010 in noise units, and the centreline steps sideways by RIFT_SHIFT each step, which is what produces the ledges.
The label is wider than the carve. RIFT_LABEL_W covers the widest carve, its swing and a wall of shell, so a column can be labelled Obsidian Rift and hold no crack.
Rifts claim ahead of the sea in the partition. NoiseBasedAquiferMixin asks the rift before the sea and returns lava at RIFT_LAVA_LEVEL with air above, except where the sea also claims the cell, in which case the cell takes the sea's water status instead.