Greymantle Caves

Those who live outside of Greymantle will be surprised to know that the vast, swampy wasteland may not possess much in the way of dry, workable land, but does house many systems of caves. They are often partially flooded, but they are still an important feature for the creatures and peoples living in Greymantle.

Caves can be part of wetland systems
Crooked Swamp Cave in New Jersey Batu caves? "swamp cave" Poison caves (black dragon?) https://www.washingtonpost.com/kidspost/2022/02/21/cave-diver-natalie-gibb/ Caves in Everglades (Palma Vista Cave?) Karst landscapes
https://keckcaves.org/can-caves-form-in-swamps/ https://en.wikipedia.org/wiki/Karst