This is the 13772nd most frequent Polish word.
podziemne
"Podziemne" means "underground" in English.
Here, 'podziemne' is an adjective describing the mines, denoting their location under the ground.
Kopalnie są podziemne.
The mines are underground.
In this sentence, 'podziemne' characterizes the tunnels as being located below the surface underground.
Podziemne jest labirynt tuneli pod miastem.
The labyrinth of tunnels under the city is underground.
Here, 'podziemne' modifies 'sources' (of water) to specify that these are found below ground level.