I really like the title of this book.
I suppose there are a few connections between this title and the actual story.
Literally, Danielle’s father was a “tunnel rat” in Vietnam during the war which, as you can probably assume, involved him crawling though dirt tunnels below ground (Danielle later visits Vietnam and crawls though a such tunnel) (and these tunnels scare me, as does Vietnam).

More metaphorically- Danielle’s life: choosing to live with her drunken, traumatized, mostly heartless father, living her childhood and adolescence in bars hearing war stories and about her father’s sexual escapades.

So, in the end, I really liked the title of the book.