This is a hard book to review because how you engage with this book and respond to its style and contents will be very personal to you and your experience. The book is poetic and philosophical, ranging over dozens of topics in a fluid, almost stream-of-consciousness, manner. For some, this style might be frustrating. Macfarlane doesn't linger on a topic; he circles around it, gently brushes by it, teases you with ideas and concepts. There's infinite space between him and the reader for both parties to have their own space, their own thoughts and feelings. Throughout the book felt as if Macfarlane was constantly walking ahead of me, but turning back to extend his hand, guiding me through the path he cut. 

What is this book actually about? Well, caves. Kind of. Caves, icebergs, tunnels, mushrooms, space, World War Two, economics, politics, history, the Paris catacombs, religion, underground rivers, mining...  It is about everything while not being about one specific thing. Every reader will have a difference experience; if you are re-reading this book, you will come away with an entirely new experience. My copy is page-flagged and highlighted, but I believe that if I re-read the book in six months I would find entirely new quotes and entirely new experience all together. 

All that to say, this was a beautiful experience that will stay with me for a long time to come.