>>9537852In Irish mythology the Aryans (blonde god people from across the sea called the Tuatha Dé Danann) rule the underworld after brokering a deal with a second race who came to the island. The second race was allowed to rule the over world while they went beneath. I highly highly doubt there’s hollow spaces beneath the earth more than simple cave systems but the mythology checks out at least. Could be an entry under newgrange idk. Also under the ice in the North Pole is an entire continent with lakes and rivers so it’s not implausible for a civilisation to be built their I just highly doubt the Germans did so.
Tuatha Dé Danann:
>https://en.m.wikipedia.org/wiki/Tuatha_Dé_Danann