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Мермерна пећина, trsl. Marble Cave in Kosovo.
It is an carst limestone cave

whose crystallized stalagmites & stalactites have formed over millions of years and also inhibits an underground lake.
The Marble Cave got its name after the metamorphosed limestone it was formed of, with the oldest rocks belonging to the oldest Paleozoic Era since the forming process continued for 200 million years.
The surroundings were composed of Paleozoic marbles that can only be found at the Eastern edges of the Kosovo Valley.
These marbles are covered with ancient volcanic lava
Later on the whole region was overflown by the Kosovo lake that has withdrawn lately. Some parts of the ceiling are decorated with stalactites in forms of organ pipes & transparent curtains. 
All units of the cave have complex networks & literal labyrinths of channels, hallways, and halls, distributed at several horizons. 
The abundance of all types of cave waters, from condensed & dripping to flowing & stagnant, adds to the beauty of the cave. 
There are 29 clear permanent lakes most of which formed in compact marbles, and only a smaller number in cave clay. 
All types of multi-colored cave jewelery can be found in the cave, from pearl-white to blue to yellow to red. 
The most numerous are stalagmites and stalactites, massif calcite pillars, cone-shaped flow-stone, drapes, sinter pools translucent tubular jewels. Due to the variety & special forms of relief & rare aragonite jewelry, Marble Cave is the most important of its kind not only in South Serbia but in the Balkans area.
Large part of the Marble Cave is still unexplored due to river deposits that block the passages. 
The snow in the river basin greatly influences the surface & ground waters, especially the underground flow & lake system in the cave.