So this is gonna be my theory on where the ancient civilizations of the world went. What if when the young Andreyas comet hit and everyone thinks it wiped out the advanced civilization from back then, but what if they saw it coming and went underground. There is plenty of evidence for this from the city's that they have found underground that have been abandoned. They probably went deeper underground for the foreseeable future until the earth's surface could become habitable for them again. Down there they evolved and became the "aliens" that we see all the time coming out of the ocean and mountains and things like that. All the aliens we see were originally supposed to be like the Mars rover and collect soil samples and stuff and do tests to see if they could come back to the surface, but when they saw people they were astounded because they thought all life on earth was gone. Now I think they kind of evolved from being underground for so long; hence the grayish skin and everything odd reported about them. Now I think we evolved from a monkey species or another hominid species or just some aliens who refused to go underground and somehow survived and we are their descendants. And I think we evolved from something because there is now evidence that monkeys are going into the stone age. Sorry this is all over the place and not well written but I wrote it on my phone at 2 am but what do yall think?

Edit: also I think they knew the comet was coming because they buried their cities.