The Under Dark Is Real - sort of . . .

Ancient tunnels reaching from Scotland to Turkey have been discovered.

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

The tunnels are believed to be over 12,000 years old. They have been found beneath hundreds of neolithic sites.

From the brief article:

'Across Europe there were thousands of them - from the north in Scotland down to the Mediterranean.
'Most are not much larger than big wormholes - just 70cm wide - just wide enough for a person to wriggle along but nothing else.
'They are interspersed with nooks, at some places it's larger and there is seating, or storage chambers and rooms.
'They do not all link up but taken together it is a massive underground network.'

'Across Europe there were thousands of them - from the north in Scotland down to the Mediterranean.
'Most are not much larger than big wormholes - just 70cm wide - just wide enough for a person to wriggle along but nothing else.
'They are interspersed with nooks, at some places it's larger and there is seating, or storage chambers and rooms.
'They do not all link up but taken together it is a massive underground network.'

Apparently these tunnels had the church freaked out and to counter the "pagan influence" they would often build chappels near the tunnel entrances. Some of the tunnels appear to have rooms with seating and other features that might have been shrines.

While there is not really anything conclusive about there having been links between them, they sure do make for a bunch of small dungeon sites all over the place. So much for the arguement that dungeons being everywhere isn't realistic. Hah!