Is there anything that suggests they're authentic, or to the contrary, anything that conclusively proves this is a modern forgery? I'm not interested whether or not this hidden land really exists, just if it's possible that these were created mid 20th century?
Nazi maps believed to be instructions for reaching Agartha have supposedly been corroborated by a letter from a German U-boat navigator named Karl Unger, who claims U-209 made it to Agartha and that the Earth is in fact, hollow. The letter also mentions the notorious German Gens. Karl Haushofer and Rudolf Hess, who he says were correct about the hollow-Earth theory.
Transcription from http://www.imperialgermans.com/documents.html:
DIVE AT THE POINT WITH THE COORDINATES: EXACT INTERSECTION: 68° SOUTHERN LATITUDE AND 1° EASTERN LATITUDE, AT A DEPTH OF 400 METERS.
The instructions should be followed strictly!
Dive. From the point of diving with half drive, a starboard slant of 10° with a bow load inclination angle of 5°. Distance 188 nm (nautical miles). The allowed depth of 500 meters. (during the driving in the corridor the pressure on the ship’s hull during maneuvers is marginally.)
Emerge. Ballast with a stern trim, emerge in an angle of 23° with a portside slant position of 22°. 190 meters upwards. Distance 75,5 nm.
Difficult maneuver! Emerge. Ballast with a stern trim, emerging inclination angle of 41° Drive straight forward. 110 meters upwards, distance 21,5 nm. Afterwards a starboard slant position of 8° until you emerge on the surface of the cave. Distance 81 nm.
Drive on the surface of the cave with a starboard slant position of 8°. Distance 286 nm.
Difficult maneuver! Dive. With a bow load, inclination angle of 45°. Until a depth of 240 meters, distance 60 nm. Afterwards with a portside slant position of 20°,at which the diving until 310 meters until the drive in the corridor is continued. After the mark of 310 meters is the diving to be proceeded with a port load capacity. Diving angle of 7° until 360 meters. Distance 70 nm. Further starboard slant position of 31° until a depth of 380 meters.
Dive. Port load, emergence angle of 22°. 199 Meters upwards with a portside slant position of 26°. Distance 43 nm.
Emerge. Stern trim. Emergence angle of 45°, always straight forward, until you breach the surface in Agharta. Distance 70 nm.
Drive to Agharta. Full speed ahead. Look straight forward, until the new light is put out. Changing of the magnetpoles. The movements of the compass needles and other meters are to be ignored. (Further instructions are in pakage Nr. 3. Open first when you arrive in Agharta!)