Ancient Mars

Alright, I have two potential interpretations of this, one more literal than the other.

1) The sci-fi fan in me is telling me that this is about a long-lived alien refugee who came to Earth from Mars before it became the dead planet we now know today. He/she is remembering a time when the planet was still alive and breathing, but there were signs (the "rusty rocks" and "pyramids") that things would not last forever. He/she likes to think that one day, a return to Mars will be possible but ultimately knows that it's not a possibility.

2) The more introspective, literary side of me is telling me that this is about a failed relationship that the singer had some time in the past, possibly in his college years due to the reference to "campus library aisles". He wants to go back to a time when he was happy in that relationship. When he was in it, there were signs that it would go badly, but he feels nostalgic for the good times. But eventually, perhaps due to bad feelings or simple drifting apart, that relationship came to an end.

Either way, the general theme of the song revolves around the fact that all things come to an end. People, places, events, civilisations, relationships, everything. Things happen and then they're over, falling victim to time and nature. In the moment, these things seem like they will go on forever, even though somewhere deep down you know that they can't. Once their time passes, nostalgia and longing for these people and places will rise and you'll want to somehow recapture your old feelings even though that's impossible. The only thing you can do is accept things for what they are - ultimately temporal and fleeting - and not forget about them, but learn to move on when they fade away.