Could this underground civilisation have areas underwater or is it another group? There are stories that UAOs in our seas that have been seen by Navy and Military. In the video above stories emerge of things seen in the oceans of the world.
There are stories of 'Gods' emerging from the seas and teaching culture, mathematics and so on... Where did these stories come from?
I think we are looking in the wrong place if we wish to follow the thread of evidence that could lead to an answer.
We are terrestrials - they would be extra-terrestrials as in extra to this earth. Words mean a lot in these things.