@djsumdog
Here's an interesting though: what counts as an extinct civilization here? Would all humans have to die to call our civilization extinct? Or would we be deemed an extinct civilization even if we merely forever lost access to advanced technology, the thing that would give civilization status at the galactic scale? (imagine we simply run out of the resources needed to build that technology, and we're unable to find alternatives).

Cause I'm starting to think that the latter could be a reasonable possibility when it comes to alien life. Was watching a report earlier that it was predicted that we could run out of helium by the end of the century. Why is that important? Because it's used extensively during chip making these days (among other high tech uses). How fast would our civilization collapse if we suddenly were never able to make CPUs at least as powerful as current Ryzen or Threadripper?
There are countless rare elements our technology depends on, and running out of even a single one could radically change society. For the most of them, you at least can argue you'd spend whatever energy needed to recycle them. But for helium... when that escapes into the atmosphere, it's gone.

So imagine the galaxy, filled with 10,000 worlds, where civilizations maybe even more advanced then us existed, but they still couldn't travel between solar systems, they couldn't gather up the resources of other worlds, and once they ran out of critical resources on their world, they slowly reverted back to a feudal society. They could be even more, 100,000, 1 million, 100 million, and we'd never know, because their small window of technological prowess, of being able to communicate via powerful radio signals, has already passed long ago, before we discovered steam engines even.

How tragic would that be? A galaxy filled with beings as smart as us, but without the resources to actually put their intelligence to use.

@djsumdog In the last few years I keep hearing of archeologist discoveries that push back when various species of humans started, or when they left Africa, so it would not be impossible to cram in somewhere a new undiscovered civilization. The difficulty is, where are the artifacts left by the civilization? Clay figures, pots, writing in clay tablets, various tools, these are the traditional signs of civilization.

Even with massive floods, at some point you'll find something scattered somewhere because archeologists and geologists have been able to identify when and where flooding took place. So if you assume a flood wiped across a valley, you'll just go downstream, where the water would have settled everything it carried, and start digging for signs of the wiped civilization.

We could of course also be dealing with tribes that for one reason or another have their "technology" based mostly around biodegradable material. Less usage of stone, mud, clay, and more wood and plant matter, that would decompose and leave nothing to find for archeologists. They could still have had language, stories with basic philosophical concepts behind them, but expressed orally. You could have had civilizations that were intellectually advanced, but at the same time, you wouldn't be able to find evidence for it. At which point it would be like believing in Santa, with no evidence.

I've heard of Göbekli Tepe only in passing, and I just know that there's some fringe ideas about it being something, with little evidence to prove it. A geographical feature that looks as if it could be man made, but no human artifacts discovered.
I don't think I've heard of the Richat Structure.

I'm open to the idea, but ultimately it's important that we find conclusive evidence of human settlement. Even with cave men we've been able to find wall art and ash remains of fires lit by humans.

I've heard of Göbekli Tepe only in passing, and I just know that there's some fringe ideas about it being something, with little evidence to prove it.

Tepe is a full on site with carvings/drawings and structures that point at a specific star, and the site appears to have been in use for over a century since newer chambers are slightly rotated to where the star would be years later. It's a little sensationalized, but the documentary Ancient Apocalypse by Hancock goes into a lot of the theory.

Even with massive floods, at some point you'll find something scattered somewhere because archeologists and geologists have been able to identify when and where flooding took place.

So Hancock presents the theory (and a lot of others support this, but it's still considered fringe) that it was multiple comet impacts. Since it was at the end of the ice age, the massive glaciers would have prevented the actual craters, but there appears to be other evidence in sediment around the world consistent with a large impact. He suggests massive floods, in a matter of days, not months, and that what we see today by satellite are the remnants of an ocean that swept over Africa.

You might be confusing Tepe with Richat, as the Richat is a series of concentric circles in the middle of the desert. There hasn't been a lot of excavation, but even what has been done hasn't revealed anything. So it could just be a natural formation, or that cataclysm was so insane it wiped out nearly everything.

We look at our layers of pavement and concrete and massive steel super structures and it seems like evidence of the remains should be here 50,000 or 100,000 years from now. At least some of it should be a sediment layer a million years from now too, but it would all likely be gone at 1 billion years. Would a single meteor strike remove all of that in <100 years? What about 5~10 of them at once? It's pretty difficult to judge.

We still have no idea how the Egyptian pyramids were built in that time, or many of the other pyramid structures in other civilizations. Roman concrete technology was lost for a while too. People debate today if King David was even a real person.

The more I look at history, the more it's such a limited black box we pull out ideas from, based on the little that survived.

@djsumdog
>Tepe is a full on site with carvings/drawings and structures that point at a specific star
I might have been thinking of something else then. I'm just familiar with the name in passing, but history was never my thing. Probably associated the name with something else by mistake.

>that it was multiple comet impacts
If you have something comet sized that hits in an ocean, you'd get a flood, and maybe you can escape rock material being ejected and all that. If it were to hit on land, ice or no ice, not only would it form a massive crater (and even in the ocean it's likely gonna form a crater, the Chicxulub crater is underwater), but it would be a fireball mass extinction event, with little chance for something over the size of rodents to survive. Chicxulub was just an asteroid, and it was enough to put the final nail in the coffin for dinosaurs (the current understanding these days is that dinosaur population was actually already in decline by that time, the asteroid just speed things up). A comet would be far worse. Multiple comets... My point is, I don't think humanity would have survived something like that. A comet wouldn't just destroy a civilization, it would reset life completely.

This is why I mention floods in general, because flooding does occur all the time, and it can occur for all sorts of reasons. You don't need exotic events like comets. I remember watching a video on some ancient flood triggered from a land slide or something like that. A "simple" flood, while less fancy, could still locally eradicate a civilization, so it's something that remains plausible. A comet impacting Earth in the last few hundred thousands years, is not really plausible.
(I was looking for something else, and found this, relating to the Younger Dryas period https://www.youtube.com/watch?v=1uEvL9cbze4 )

>You might be confusing Tepe with Richat
No, I think I'm confusing it with Gunung Padang. (relevant info:
https://www.youtube.com/watch?v=zU-wQVAqQnk
https://www.youtube.com/watch?v=OZGp6N3AHTA
)

>We look at our layers of pavement and concrete and massive steel super structures and it seems like evidence of the remains should be here 50,000 or 100,000 years from now.
Some of it might erode even faster, but I don't think that would be the most categorical thing that would date our civilization.

>At least some of it should be a sediment layer a million years from now too, but it would all likely be gone at 1 billion years
Yup, what you'd want to look for are the sediment layers. And once material gets deposited in a sediment layer, it will stay there. Even for a billion years. But asphalt and concrete might not be as noticeable in between all the other rocks. And steel will eventually rust away, so you'll just have the sediment more rich in iron oxide. But that wouldn't necessarily be indicative of a civilization.

Some of the things you'd want to look for in the sediment layers to discover humanity is microplastics and artificially created nuclear isotopes. Actually, I remember hearing somewhere that they looked in the sediment of some lake or reservoir or something, that hadn't been disturbed by humans for a long time, and they discovered that we, as in humanity, had already left a clear mark of our existence in the sediment. And I think it was mostly about nuclear isotopes that don't exist naturally. Lead is gonna be another strong sign, cause asteroid don't deposit lead when they strike. So an uniform distribution of lead in the geological record could only have been caused by us.

>We still have no idea how the Egyptian pyramids were built in that time
No, we pretty much know. The thing is, everyone who insists "we don't know" think about the problem in short time spans, they think the pyramids should have been done in 3-5 years, like a modern skyscraper gets built. But Egyptians spent decades for even small pyramids. Cutting the blocks was done with sand abrasion, moving them was probably done with the help of rollers beneath the blocks (or maybe this https://www.livescience.com/45285-how-egyptians-moved-pyramid-stones.html), and for lifting them into place they would have used sloped ramps. Sure, it might take multiple days to cut, move and place a single block in the pyramid. But Egyptians had plenty of time, and slavery ensured they had man power too.

>The more I look at history, the more it's such a limited black box we pull out ideas from
Oh, I'm sure there's plenty we don't know. Even for something like the Roman Empire, that was likely one of the better documented old civilizations, I bet we don't have a clue about the majority of their politicians and local leaders. But we do know that the Roman Empire existed. By this point, it's really hard for something of a big scale to have eluded us noticing. We're at the point of refining details.

Coming back to the dinosaurs as an example, we know they existed, we know they died, we know they died after an asteroid strike, now we're refining how much the asteroid is responsible, and how much is it other changes in climate that had been taking place for other geological reasons.