I have a question about ancient ruins.

[PoindexterOglethorpe]

2. Also, things fall down.

It depends somewhat on the materials used in the first place, and whether or not the building or city was completely abandoned and no one ever went there again, but over decades, hundreds, and thousands of years, things crumble and fall down.

The time span for human built things is relatively short, just a few thousand years, but over hundreds of thousands or even millions of years, absolutely nothing would be left. I've recently read something about that, which pointed out that on about a two million year time frame, we would not be able to detect any kind of artificial structure. Which means there could theoretically have been a technological civilization as recently as two million years ago, and we'd have no way of knowing about them. Let alone, say, a technological dinosaur civilization.