I liked looking at these underground city tunnels to feed my imagination with spooky places when I wrote the underground tunnel scenes in The Grotto's Secret.
Exeter's underground tunnel inspired me when writing my crisis scene in The Grotto's Secret where my heroine flees from the bad guy. You can read the full story about Exeter's underground tunnel here.
Here's another interesting tunnel.
En.Wikipedia explains this image: A typical view from inside the underground city in Derinkuyu, one of the largest underground complexes in Cappadocia.
There are few artifacts left from the original builders, mainly just large rocks that were used to block the passage for intruders.
Most of the "cities" are corridors, but some places there are rooms large enough to live in, and some rooms that have the cross-like shape of a church, which is probably exactly what they were.