https://www.youtube.com/watch?v=U1QrTsoNMYM
Its a Paranormal_World whenever you encounter something that isn't easily explained through conventional perspectives.
https://www.youtube.com/watch?v=tzVAPAluxs4&feature=youtu.be
This story has been drifting around for a while but its still interesting, given the potential implications.

http://www.tcm.com/tcmdb/title/Five-Million-Years-to-Earth-
https://www.youtube.com/watch?v=v5xPvFjPhkQ

An Ancient_Aliens documentary on Ancient Tunnels, of course.
https://www.youtube.com/watch?v=YHIBCBmC8k0

Small tunnels such as the ones around the Ellora-Caves in India are quite perplexing.
https://www.youtube.com/watch?v=8w8FxWtDKJA

32,000 Year Old Alien City Discovered in India

https://www.youtube.com/watch?v=SF9MZsyq5o8

Dwarka: Atlantis of the East
https://www.youtube.com/watch?v=NVIsjx5X3QM

http://www.dailymail.co.uk/sciencetech/article-2022322/The-massive-European-network-Stone-Age-tunnels-weaves-Scotland-Turkey.html-
https://www.youtube.com/watch?v=C2Tak7uEYJ8

Whether its in ancient times or more modern times, building things underground certainly has its advantages.

Massive Underground developments at the City of Helsinki
https://www.youtube.com/watch?v=jXNyEiw28D0

Secret Humans, Giant Nephilim, Aliens, Ancient Annunaki Sumerian Mesopotamia; (Forbidden Archeology), Giant or Alien or Annunaki...
https://www.youtube.com/watch?v=_d5oJsprjhM

Shocking Documentary: Ancient Cosmic War : Lost Civilization: Amazing UFOs & Aliens...
https://www.youtube.com/watch?v=_gToQm-Kvr8

Erich Von Daniken Lost Technology and Ancient Aliens