camouflaged, underground or just hidden in plain-sight ?
In 1981 I visited the Seegrotto near Hinterbühl just outside Vienna, Austria.
The caves are on basically 2 levels -- the Germans blew the bottoms out of the upper caves, draining the water into the lower few. They then concreted the floors of the upper & several lower ones installing drains to keep them dry. Post war the drains were blocked & the caves allowed to refill naturally -- you can now take a shallow-draught boat trip thru the upper ones.
History ...
http://www.microsofttranslator.com/bv.aspx?ref=SERP&br=ro&mkt=en-AU&dl=en&lp=DE_EN&a=http%3a%2f%2fde.wikipedia.org%2fwiki%2fSeegrotteBelow are 2 postcards that I bought there ...