We've found evidence of humanoid bones that are clearly much larger than those of a normal person, dating from hundreds to thousands of years ago, all around the world. Which would certainly explain why nearly every culture has some sort of myth regarding giants, or how there supposedly exist firsthand accounts from explorers who have claimed to visit and interact with giants. Or how supposedly some ancient structures were made especially large seemingly to suit certain 'giant' individuals. Though while each time the bones themselves were found they were considered hoaxes, it doesn't discredit the other evidence found.
The underground cities come from reports of cities that were built underground. Which includes stuff like Derinkuyu but there also exist stories of other cities, some of them being from unknown civilizations. Which brings me to the last point of civilizations before Babylon, which mainly comes down to the fact that realistically there's no way that Babylon was the first real human civilization. Especially when you consider just how long humans have been on Earth, and that even looking at Babylonian architecture it likely took hundreds of years for us to even reach that point. Hell, the first known piece of literature we know starts with the narrator reminiscing about "the good old days" so it's super likely there were civilizations from before Babylon that people were getting nostalgic about. Not unlike how some people today get nostalgic about stuff like Rome.
Julius Caesar comes in when, as part of his orders to burn the boats docked in the port of Alexandria, accidentally set fire to the city and therefore the library itself. Of which the library was created partially to hold information from kingdoms considered ancient even by the standards of ancient Egypt and Greece. We don't know for sure that it had information on any of the above points I mentioned, and we never will. But what's undeniable is that human history, when you go far back enough, has a lot of holes that, while they could have been filled at some point, are likely to now remain empty forever.
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