How would human civilization look if it had survived for a billion years? This is the key idea Clarke poses and explores in this novel. It is well written and proposes some believable answers to that question.

The story centers around the billion-year city Diaspar, which was built to last for eternity. At its core, it builds on stagnation, but with constant permutations to give the feeling of change. To me, Diaspar is the brilliance of this book, and most things about it—from its inhabitants to its engineering—are interesting to think about.

Without spoiling too much, I found the part about Lys the most disappointing. I don't think it adequately explores how such a civilization could survive for such a long time. Maybe I just need to find the answer for myself, but I would have loved more hints or ideas for such a different culture.

This is certainly a very good science fiction novel. It reads extremely modern for how old it is. I think the foundational ideas of eternity and Diaspar will stay in my mind for a long time, but a lot of the actual plot feels slightly less interesting.