Link:

Featuring rare location hi-definition footage, The History Channel shows a world filled with mysteries and secrets just below the ground we walk on. Full documentary playlist: 20 hours.
There are 27 episodes in the series starting at:

1. Cities of the Underworld - Istanbul
2. Cities of the Underworld - Scotland's Sin City

... through to...

26. Cities of the Underworld - Catacombs of Death
27. Cities of the Underworld - Secret Soviet Bases