Should it exist, the booty would certainly be vast. During World War II German troops looted the bank reserves of conquered countries and took the gold back to Germany.
Victims of the holocaust were also stripped of any valuables they had, including gold jewellery. The gold from these sources was then melted down and cast into bars with the mark of the German central bank, the Reichsbank, imprinted on them.
Much of this loot was used to pay for the war effort, but a large portion was still intact and in Nazi hands as the end of the war neared.
By April of 1945 the Allies were closing in on the German capital and Nazi officials decided to move the remaining contents of the Reichsbank, ostensibly to Oberbayern in southern Bavaria.
It never reached there - but [local historian Hilmar] Prosche and his backers believe it lies deep beneath the earth.
The story is worthwhile not only because it aims at more closure for one of history's greatest evils, but also because the techniques used can be transferred to mining. To take a single example, secret artisanal mining areas can be found using the photographic scanning technique.
It's possible that the photo-scanning was adapted from mining companies already using it. If so, then some good came out of the technique over and above finding promising gold deposits.