This is the 13301st most frequent Russian word.
мраморном
"Мраморном" is the prepositional case, singular form of "мраморный," meaning "marble" (adjective, e.g., "made of marble" or "marble-like").
In this sentence, 'мраморном' is used as an adjective describing the material of a building.
Я прошёл мимо мраморном здании в центре города.
I passed by the marble building in the city center.
Here, 'мраморном' refers to the floor's material, indicating its composition.
На мраморном полу был изысканный узор.
There was an exquisite pattern on the marble floor.
In this case, 'мраморном' describes the grotto's characteristic as being made of or containing marble.