While looking into Underworld/Hollow Earth Theories it would be 
criminal not to take notice of four books which were written as 
fact ... stories of travel to underground cities.

Darkness over Tibet (1937) by T. Illion.

According to the info on 
the back cover, "Darkness over Tibet [is] "The First authentic and 
fully detailed account of an underground city in Tibet by the only 
westerner ever to visit there and come back and tell about it". 
David Hatcher Childress reissued this well documented book in his 
Mystic Traveller Series thru the Adventures Unlimited Press.

Beasts, Men and Gods (1922) by Ferdinand Ossendowski.

 The author 
tells of his flight across from Russia to escape from those who would 
harm him during the Russian revolution in 1920 and his travels all 
across Asia during that period. The last part [5] tells of his 
adventures in the Subterranean Kingdom of "The King of the World." 
Lewis Stanton Palen tells us in an included "Explanatory Note":
"When one of the leading publicists in America, Dr. Albert Shaw of 
the 'Review of Reviews', after reading the manuscript.. ..[wrote] ... 
the succession of trying and thrilling experiences recorded seems in 
places too highly colored to be real, or, sometimes, even possible in 
this day and generation. I desire therefore, to assure the reader at 
the outset that Dr. Ossendowski is a man of long and diverse 
experience as a scientist and writer with a training for careful 
observation which should put the stamp of accuracy and reliability on 
his chronicle. 

Eyes of the Superstitions (1993) and

 Beneath the Superstitions (1995) 
by Kaye Kizziar concerning her personal knowledge of UFOs and 
underground bases in the mysterious Superstition Mountains of Arizona.
 Enjoy!
  Dennis

T