In 1988 my husband and I took an American Express trip to Austria and Germany. It was partially escorted, they set up your transportation and connections, but you basically could arrange to do all your own things.

We went first to Munich, then Salzburg, then Vienna. In the outskirts of Vienna in Hinterbruhl, we visited the Seegrotte which was quite a fascinating place. The idea of it seems kind of far fetched to some. It is however, the largest subterranean lake in Europe. It was once a chalk mine. During WWII it was used for the development and production of the German jet fighter plane, Heinkel HE 162, the so-called "Volksjager" (people's fighter).

 The lake grotto is a former gypsum mine on 3 levels; grey and red gypsum were mined here for use as agricultural fertilizer from 1848 to 1912. In 1912, over 20 mil. liters flooded the lowest level and the mine was closed.

 In the 1930's a team of cave explorers found the mine, with the lake in it. It was reopened in 1932 as a display mine for tourists. During WWII a subterranean airplane factory was established; the installations were demolished in 1945. This was established underground to protect the factory from bombing. It was destroyed by the German Army to prevent it from falling into Allied hands.

You enter the mine right in the area of the Seegrotte Cafe. It had at the time some very good wurst and bier, also strudel.

The walk into the mine is a bit long. I remember that there was a shrine to St. Barbara there. She is the patron saint of miners. Masses in her honor have been performed here with over 3000 guests attending.

Photo here courtesy of A. V. Luik 

As part of the end of the end of visit you make a short boat trip; the boat is powered by underwater electric engines. It glides smoothly and soundlessy over the water and offers you a spectacular perspective over the illuminated canals and enchanting reflections in the water.The boat trip itself was very calming. I seem to remember some classical music playing, but it was a very long time ago. I remember it being very pitch dark. You stood on a kind of small wooden pier and boarded the boats. They traveled very quietly by electric motor.

Apparently in 2004 there an accident and one of thes boats sprung a leak and while most people were rescued, 4 were not and drowned, becoming somehow stuck  under the boat. The boat trips were cancelled then for some time, but you can now participate more safely today.

During the war, all the water was pumped out of the mine. Seven natural springs pump in 60,000 liters( 15,000 gallons) a day and there is no natural outflow, therefore even though it is a lake, it is pumped everyday to maintain it's boat channel to a level of 1.20 meters(about 4 ft.) deep, although the lake measures 12 meters(39 ft.) at its deepest point. Also during the war, warm air was pumped into the mine to make it a warmer temperature. The warmest gallery in now 12 c(54 f.) and the normal summer and winter temperatures are 9 c(48 f). 

Prisoners from the sub camp Hinterbruhl of Mauthausen Concentration Camp were part of the slave labor force in the mine that was producing the Heinkel fight jet fuselages, the HE 162, a Nazi 'secret weapon'. Also working here were POW's.

 Photo here courtesy of A. V. Luik 

 http://www.seegrotte.at/index.php?id=14&L=1

This website has some awesome pictures, this is the official site which takes you through both the upper and lower level of the mine.

 When Disney made the film “The Three Musketeers” they used the underground lake as a film location. They built a large Viking like vessel and filmed the movie on board to simulate them sailing under the Bastille as they went to rescue the King, the boat has been left behind and makes an impressive sight sitting in the water. This is the reference you hear in the Japanese you tube video and also what you see in the grotto on the film. This was obviously after my visit there! It was filmed in 1993.http://www.imdb.com/title/tt0108333/ ( Charlie Sheen, Kiefer Sutherland, Chris O'Donnell)

In the last days of the war in 1945, the inmates of other camps had to make a 200 km-long march to the concentration camp Mauthausen in Hinterbrühl. Virtually none of them survived. Fifty-one inmates were killed even before the march by a gasoline injections or strangled by SS officers. In 1988 a monument was erected above the Subterranean Lake to honor the 51 victims of this massacre. 

 http://en.wikipedia.org/wiki/Hinterbr%C3%BChl

The above and following pictures and illustrations were from a guidebook that we purchased on that trip in 1987. The above picture shows the Heinkel 162 jet plane and also a scene from the factory after the explosion, which is the one I presume that occurred when they did not want the Allies to have anything from the factory and tried to destroy it.

From the entrance of the mine, you can see what is thought to be the first plane assembly being hauled out.

Here is the underground assembly line.

Here are workers, POW's, slave labor from the camps and others.

The Bottom of the first fight jet assembly. 

Completed plane. These planes were metal, which was scare and mostly wood. The location of the engine made it necessary for the first ejection seat to be build as any pilot bailing would have had his shute caught in the engine. The plane was designed to be used by Hitler youth as the fleet of pilots was no longer viable, having been mostly killed by this time. The engine on the top caused difficulties with unstable pitch. The plane was literally glued together in many places and with failure of that and others things it proved to be more difficult to fly and required an expert pilot. The Hitler youth were then to get a version which was more of a glider and begin training in March of 1945 in Sagan. That did not happen. It seems that only one or two of those gliders ever hit the air.

The jet did do some combat time in April 1945, but due to structural issues, fin and rudder assemblies broke off, amongst the issues with glue and the nose of the plane and various other problems it was never a significant problem for the Allies in battle, nor a deterrent. 

Diagram of the mine.

Boats back in the day we were on them.

Entrance to the subterranean world.