From the Brinsley Le Poer Trench book, page 61:
(Lest we forget!)
"Among the Mongolian tribes of inner Mongolia, even today, there are
traditions about tunnels and subterranean worlds which sound as fantastic as
anything in modern novels. One legend, if it be that, says that the tunnels
lead to a subterranean world of antidiluvian descent somewhere in a recess
of Afghanisthan, or in the region of the Hindu Kush
"
"It is even given the name Agharti. The legend adds that a labyrinth of
tunnels and underground passages is extended in a series of links connecting
Agharti with all other such underground worlds!
The subterranean world, it
is said, is lit by a strange green luminescence which favors the growth of
crops and conduces to length of days and health."
"This last account is of special interest because Kolsimorefers to this
green flourescence in another part of the world. He writes in Timeless Earth
about a strange "bottomless pit" in Azerbaijan in the Soviet Union.
Apparently, a bluish light comes from its wall and odd nioses are heard.
Eventually, after investigating and exploring, scientists found a whole
system of tunnels connecting with other ones in Georgia and all over the
Caucasus."
And speaking of bluish and greenish luminescence:
The Green Children of Wolfpitte:
http://www.holloworbs.com/green_children_wolfpitte.htm