I love the idea of lost civilizations and dead cultures in my worlds - both real and imagined. There's something inspiring about knowing that once this world of ours held people who were far more technologically advanced than we would have believed possible at the time and that they suddenly decided to fuck off and disappear. Think about that for a moment. It would be like waking up one day and finding out that French had suddenly decided that they had run their course and just disappeared into the world, leaving behind empty cities and monuments for the rest of us to stare at in mute horror.
What's wild about that is that it's actually happened in recorded history. The Minoans, the Maya, the Mississippi Valley Mound Builders, and more thrived during their day, often dominating their regions of the world, and then they just disappeared. Sometimes we can link back the cause to natural disasters (I think they blame the Mayan collapse on a pair of century long droughts now), but more often than not we're left with nothing but a bunch of pictographs, ruins, and countless questions.
I love it. I love it because it reminds me that no matter how awesome we believe we are it could all be gone tomorrow. Then a thousand years later comes along and our descendants who excavate the holy remnants of Las Vegas out in the desert and wonder why our gods all liked so many lights and bells (of course the like sacrificial money, but all that noise!).