This is "The Tube Cape" and island in Russia where strange formations of stone were found. Traditional geologists think that it might be lava structures, but locals who live nearby, and many visitors who come here from different parts of Russia doubt this and call them man
made, probably belonging to some unknown people who lived here in prehistoric times. Some new-agers even suppose that this structures were used by some technologically advanced civilization that existed on the Earth at the age before the Bible flood.
Two Moscow inhabitants decided to explore collecting system under the building where they live. So they found the right gully grating, opened it and came down. How they were surprised, discovered the whole system of caves with the real underground river.
Now they say, that it is one of their favorite places in Moscow to have a walk at evenings, or just to have a rest near river right under the house. See also: Moscow Underground Tunnels (HDR) Moscow Sewerage Moscow Dungeons