Looking for a book like Tunnels (2007)

I read Tunnels by Roderick Gordon years ago in my teens and still remember how much I loved the detailed underground world – caves, rivers, hidden systems, and the sense of exploring an entire civilization beneath the surface.

Lately I’ve been craving a fiction book with a similar atmosphere: stories set in subterranean worlds, cave and river networks, or underground environments that feel immersive and richly described. Adventure is great, but I’m also open to slower or more atmospheric books if the setting is compelling.

Any recommendations for novels that capture that kind of underground exploration or worldbuilding?