If using a custom world that decreases world height (while keeping the same top_slide) a lot of aquifers and cave openings are visible. I suspect this might be due to the caves not knowing terrain has actually stopped lower because it hit top_slide
[media]Custom world settings in attachments
I was able to reproduce this in 21w38a using the provided settings
But in the latest snapshot the world settings do not work I request an updated setting to be posted