Other that that a mining expedition could plausibly go to the exposed mountains and chose a promising location and then begun tunneling. Time and place constraints make even that a serious challenge. the right geology is not immediately obvious and that should take weeks if you are lucky. This presumes their earliest expedition was lucky.
Then did 250,000 Germans disappear at all? Whatever did happen, then technology was very limiting and only the building of extensive underground workings might have been practical and these could be extensively expanded over any number of years with minimum difficulty. Geothermal heat was potentially available for energy.
Such an enterprise was not completely impossible to engineer. Again supply remains completely improbable.
NAZI Flying Saucers; A Permanent Base in Antarctica
A highly guarded secret is the probablity that German Nazis, as early as the 1930s, have built a secret base at the South Pole.
I was thinking if it is reasonable to publish your manuscript concerning the U-530. The three submarines that took part in that operation (U-977, U-530 and U-465) are currently at the bottom of the Atlantic Ocean.
Isn't it better to leave them there? My old friend, think about it! Think please how then my book will look when you publish your memoirs (after, WWII Heinz Schäffer wrote a book named "U-977").
We all made an oath to keep the secret; we did nothing wrong, we just obeyed the orders and fought for our loved Germany and its survival.
Please think again: isn't it better to picture everything as a fable? What results do you plan to achieve with your revelations?
Think about it, please."