Cave Passage

Cave Passages are a natural formation that allows players to enter and exit the first Cave layer form the "overworld."

Res Communis

Even when claimed, a Cave Passage seems to be a special case that always counts as "Res communis" - it's never a Crime to enter/exit, and not even visitor buff will prevent anyone from doing so. (Almost everything else in Hearthlands is "Res nullius" which can be claimed as property protected by Hearthlaw.

Base Security

If you build a base that contains a Cave Passage, not properly fortifying both sides can compromise notjust one, but both of them. Village claim extended downwards is more secure because people using the Cave Passage technically never leave the Vclaim. (verify!) meanwhile, having two separate Personal Claims on the overworld and cave layer means Visitor Buff does not persist between them.

Naturalia

Dusk Ferns specifically only spawn within a certain distance from the cave-side of a Cave Passage.

Relation to Terrain

Cave Passage can only exist as a part of a cliff. This means they can be relatively rare in grasslands and forests, but are abundant in the ridges Badlands and all over mountains.