"Underground tunnels" refers to passageways or pathways that are built or exist beneath the surface of the ground, allowing people or things to move or travel underground. Full definition
It's definitely a walking city as the narrow streets are a nightmare for cars and one of the city's unique features is a network of underground tunnels that are used as roads. (neverendingvoyage.com)
Using mathematics and biologically - inspired algorithms, the technology uses vehicle - mounted cameras to track the location of the vehicle in underground tunnels to within metres. (sciencedaily.com)
There's an entire underground tunnel system so the cast and crew can «time travel» from one place to another. (inc.com)