Aquifers and lava aquifers can spawn in the deep dark

All types of aquifer can spawn in the deep dark, causing it to get flooded with water or lava. This can make ancient cities very difficult or impossible to explore once they get added.

To reproduce, use the seed -3945541560037994869 and run the following command:

/execute in minecraft:overworld run tp @p 1004 -23 65