New Athos Cave

For most of its known existence, the New Athos Cave was simply called the "Bottomless Pit" but it has now been revealed to in fact have limits to its depths, but they are so vast that a metro train was built to traverse them . 

The giant cave system, located inside of Mount Iberia, was known to locals before it was discovered by the world at large in 1961 when it was named after the nearby town. It was discovered to consist of nine massive chambers comprising a cumulative volume of over a million cubic meters, making it the second largest cave in the world. The cave proved to hold a number of features including underground rivers, staggeringly large stalactites and stalagmites, and deep ravines. Each of the giant sections was given a unique name and after the exploration was finished, the cave was opened to the public. Unfortunately moving people through such a colossal space proved to be an issue. 

To rectify the problem a short railway was set up in the cave system in 1975 looking not unlike a metro subway. The train, known as "Turist" travels to three stops along its subterranean route, passing through huge, unfinished cave areas between stops. There are opportunities to exit the train and explore the caves via long railed walkways, which pass by rock formations that have been illuminated by colored lights.

Not all portions of the huge caves are open to visitors, but with such a massive amount of space to try and comprehend it's hard to imagine anyone noticing that the caves are actually bigger than they can see.