I find this whole line of thought fascinating. The best explanation I've heard is that 12,000 years ago the earth was hit by a series of asteroids or a comet breakup flying through the Taurid Meteor Stream.
It wiped out a developed civilization...perhaps not developed along the same lines as ours and perhaps with different technologies. The survivors figured out it would likely happen again and so built ancient structures like the pyramids and a number of underground cities in Cappadocia Turkey in order for humanity to ride out the next one. The Taurids happen in spring and fall and so they also became fixated on the solstices and built structures to calculate and warn future humans. Randall Carlson, Graham Hancock and others paint a fascinating picture of this.
I don’t know where the poster is getting information about underground newtworks at Göbekli Tepe. AFAIK there is no such evidence—at all. As to ancient sites underwater, that’s easy to explain: we’re on the tail end of an ice age and ocean levels have risen as ice melted.
The evidence suggests an advanced civilization existing before 10,000 B.C. or more.
I've wondered about an asteroid or comet causing a prolonged winter, driving people underground.
Another idea is a war such as is described in the Mahabharata. Certainly a nuclear war would explain it, but the lack of persistent radioactivity argues against that, unless it happened in the far distant past. Civil unrest could drive people underground for protection, and that might be the explanation.
I do think the Vedic texts are history. So do the Indians. One can visit the Battlefield of Kurukshetra today.