Is it true that there are deep underground tunnels and catacombs in Russia that no one knows the origins of?

I know my grandpa used to tell me in the Carpathians in Romania there are caves that have tunnels that go deep into the Earth that people have yet to fully explore and it always made me question about whether there used to be (or still is) an ancient race of underground humans.

I was recently playing the Metro games and it gets mentioned a bunch of times that Russia also has this stuff.

Wouldn't it be interesting if this underground civilization had managed to span the globe without even worrying about sailing?

What evidence exists out there of this kind of stuff?

http://www.ancient-origins.net/news-history-archaeology/extensive-ancient-underground-networks-discovered-throughout-europe-00540

http://www.ancientpages.com/2015/05/25/11-mysterious-ancient-underground-worlds-that-remain-unsolved-to-this-day/

http://www.dailymail.co.uk/sciencetech/article-2022322/The-massive-European-network-Stone-Age-tunnels-weaves-Scotland-Turkey.html

http://www.ancientpages.com/2015/10/04/secret-ancient-subterranean-tunnels-and-caverns-across-america-who-or-what-were-our-ancestors-hiding-from/

http://b-c-ing-u.com/2015/11/09/ancient-tunnels-under-the-sea-tunnels-under-the-ocean-really/

http://www.subterraneanbases.com/mysteries-under-moscow/

What do you think, /pol/? What have you heard about the underground people?