This is the 11102nd most frequent Catalan word.
subterrànies
"Subterrànies" means "underground" or "subterranean" in English.
Here, 'subterrànies' means 'underground' and is describing the caves being below the surface level.
Les coves subterrànies són molt impressionants.
The underground caves are very impressive.
In this context, 'subterrànies' indicates rivers that flow beneath the surface of the earth.
Hi ha rius subterrànies que travessen la muntanya.
There are underground rivers that traverse the mountain.
Here, 'subterrànies' refers to constructions that are located below ground level.