New Athos (Novoafonskaya) cave is located in the Iverian mountain of Abkhazia (Georgia). It is one of the 3 largest caves in the world (largest in volume), but only a portion is open to the public. New Athos cave has its own railway system.
Abkhazia has one of the deepest (and yet unexplored) cave systems in the world, which represent an attraction for speleologists around the world. See related article here Caves in Abkhazia get deeper