Underground Rivers by Richard Heggen. A 1,500 page PDF book kindly placed online for free by the author in its latest summer 2018 draft, under Creative Commons Attribution NC. Effectively it’s a chronological and thematic encyclopedia with comprehensive coverage of British and American popular culture, and abundant illustrations. It also has many rich dips into ancient mythology on the topic. It’s evidently a years-long side-project and the author, Professor Emeritus of Civil Engineering at The University of New Mexico, asks to be informed of anything he may have missed over the decades.

While many of the illustration are in the public domain, some are likely not. It would probably be safer to assume that the Creative Commons Attribution NC licence applies to the text only.