Water or lava flows from the cave ceiling and cave walls.

The stone wall does not clog the wall, making it visible underwater, and when the block is placed, the water will flow out.

The underground aquifers do not have a barrier at the edge of the cave causing water to flow out.

Minecraft Cave and Cliff Prototype Datapack don't work in 21w16a

In the Overworld, water or lava flows out of the way, making it look unattractive or sloppy and dangerous as well.

In the Nether, lava waterfalls flowed out, making them look unnatural and rowdy or dangerous.

The caves in Minecraft 1.17 have water or lava flowing in a waterfall, which, when playing, can feel dangerous, such as being swept into a lava pool, or when you fail to walk into a lava-flow area, causing death many times.