I feel somehow touched by things that go on over an extremely long course of time.
It took around 500 years for Stonehenge to be completed. Considering the life expectancy that time, it's like more than 10 generations. I don't think the last generation of the builders would have any idea about the first, let alone us.
The same feeling happened to me when I read a Japanese comic called "Blame!". The protagonist happens to be immortal. His mission has been going on for so long that he forgets the exact purpose. How long is it? In one scene he has to walk through a room the size of Jupiter.
Recently Jeff Bezos of Amazon.com just funded a 10,000-year clock project called The Clock of the Long Now. It's just like Stonehenge of our generation. People in the future would be totally oblivious of the purpose of it.
Maybe it's not how long it takes that touches me, but how relatively insignificant life is. In Arthur Dent's words, what a sad little blip of an existence...