An amazing book. Amazing because of the varied underground adventures the author undertook throughout Europe to visit catacombs, caves and underground phenomena both manmade and natural. The book contains fascinating and moving history, science, discovery, outdoor exploits and cultural oddities. You might want to have a dictionary nearby as you read, though, because the author uses more unusual words than anyone else I've ever read in English.
I have not yet seen this book but there is an essay reviewing it in The Atlantic, p.36, July 2019, Rebecca Giggs. Add 'plastiglomerate' to your vocabulary!