[Diogenesian]

The exception that proves the rule is a nuclear reactor from Gabon that dates to 1.7bn years ago... and which was clearly natural, a uranium-rich cave got flooded and went supercritical: https://en.wikipedia.org/wiki/Oklo

Of course we haven't discovered everything. But it seems very likely that if there were a civilization's worth of slag from 500m years ago, we would have discovered it.

A more interesting question is the civilizational technology of domestication, especially since that is short-term technology coupled with long-term evolution. It does seem possible (but purely fantastical) that Earth had talking dinosaurs with sophisticated ideologies and mythologies, feeding dumber dinosaurs and curating the growth of nutritious plants, the descendants of which all died 65m years ago.

[saidnooneever]

there are many of these examples. in africa mines have been found by mining etc. its very interesting and none of it fits any current ideas. it does not prove anything definitvively but i do think if you look at billion year timescales a lot more is possible 'without any trace' than say million year timescales.

earth is very old and has had life on it for very very long, that is what we know. how exactly it looked like we're still finding out and some things we might never get a clear picture of.

[jerf]

People get way too excited about the word "reactor". They hear "reactor" and think "oh, a natural formation of uranium was evaporating water into steam and generating electricity". They don't consciously examine that, of course, but it's clear a lot of people are letting that in unexamined.

What really happened is that some uranium got concentrated, and then it did was uranium does.

Similarly, if you want to make a very, very basic atomic bomb, you can do it by picking up two subcritical bits of uranium of the right isotopes and slamming it together in your hands. Basically the demon core but even more pure. The yield will be pathetic by military standards but you won't be around to care.

Fission isn't a process that requires intelligence, and fission causing cascading fission doesn't either.

"A more interesting question is the civilizational technology of domestication, especially since that is short-term technology coupled with long-term evolution. It does seem possible (but purely fantastical) that Earth had talking dinosaurs with sophisticated ideologies and mythologies, feeding dumber dinosaurs and curating the growth of nutritious plants, the descendants of which all died 65m years ago."

Yes, that's why I'm careful to qualify it with "technological", which is by implication the whole thing we do with metals and minerals and oil and all that stuff. A past intelligent society that lived like hippies and had all sorts of fascinating philosophical debates until they died out with no major footprint can't be completely excluded. But I am fairly confident they never went to space. Well... you know... real space, like the kind of orbit the ISS is in. What they may have gotten up to on the influence of drugs I have no comment about.

Part of the reason I couldn't exclude it is that it has basically happened. If hypothetically an asteroid blew away humanity two or three thousand years ago, that's basically what would have been left behind. They did mining then, but at a scale easy to believe could be erased by time. Quite a bit of that time period is already gone as it is, let alone across millions of years. It's really that modern industrial mining that we do today, the cumulative effect of many decades of that mining, and barring disaster, the cumulative effect of yet more decades/centuries of mining that would survive for millions of years.

[mcswell]

"talking dinosaurs": I don't know about that, but the Farside had a picture of cigarette-smoking dinosaurs, which requires at least some intelligence: https://i.pinimg.com/736x/6a/87/1a/6a871af3a7754a3ac7f1b2c11...

[Diogenesian]

The first part of your comment seems to be replying to a pure strawman, and missing the point of my argument. My point was even small natural reactors leave very obvious evidence, so it seems like a city's worth of metallurgy could be detected 200m years later, probably by a surprised geologist in the 1800s. The fact that we haven't seen anything is conclusive evidence that modern humans were the first metallurgists.

  Yes, that's why I'm careful to qualify it with "technological", which is by implication the whole thing we do with metals and minerals and oil and all that stuff. 

"And minerals" is doing a lot of work, and it is just wrong to exclude plant/animal domestication from technology. Likewise earthworks, irrigation, trading networks and currency, etc. You're being very dismissive and closed-minded about what technology is. ("lived like hippies")

In particular, there don't seem to have been any dinosaurs that had anything like primate hands, so they would have never built real tools or weapons, or manipulated fires. That doesn't close off the possibility of an "advanced civilization" unless you mean "uses tools as fancy as ours." We don't actually know what 200,000 years of natural domestication would look like.

[jerf]

Ah, sorry, I did misinterpret. My apologies.

Add our nuclear waste dumps to the list of things I mention in another reply that I can't guarantee wouldn't still leave legible evidence even millions of years later. Especially since we seek out geologically stable places to put them... not that we've actually put them there yet, but, you know, maybe over the next couple hundred years we'll get it together....