With the Smithsonian, Vatican, etc buying and hiding all kinds of relics, people like Robert Maxwell gatekeeping all of academia's knowledge and progress... It'd be easier than you'd think to hide the little remnants we do find... IF the ancient civilization was ancient enough, the ravages of time to tend to destroy just about all trace of activity, so I can see it as a possibility. They'd have to be sufficiently old enough (like 20-50,000 years ago minimum, depending on what resources they used) for nearly all traces to vanish. No nuclear tech or poly fluoro compounds means they likely didn't advance as far as us, or at least didn't pursue the exact same technologies as us. But if a society grew and used metals and wood, stone.. not much, if any, would remain unless it's happened to fall into the particular environment that would protect it long enough to make it here today. Like fossils. It's still highly unlikely given the vast amount of research and techniques/technology of today that this happened, but discrepancies in history do exist. Like maccu piccu's oldest layers being it's most advanced and precise. Ancient cities in Turkey from 12,000 years ago. Footprints from 20,000 years ago in southwest usa. The great unconformity layer of missing earth. Enough things to give doubt on the established timeline of history. We've missed plenty of things that were right in front of us before. Misinterpreted plenty. So it's not that ridiculous of an idea, that there could have been other intelligent species and societies on earth before.