Maurice Gee wrote a great kid's sci-fi novel around the tunnels in 1979 called Under the Mountain https://en.wikipedia.org/wiki/Under_the_Mountain_(novel), which was a good read back in the day...

Just wanted to say, I adore Maurice Gee, "The Halfmen of O" left quite the impression on me as a child. Great quintessential New Zealand sci-fi novels from a fantastic mind.

Yeah, that was another great one too. He always struck me as being well-known in NZ, but less so outside it, which felt like a real shame.

And the children's series, which made its way to the United States during Nickelodeon's bundle series, The Third Eye.

Although it's from a "newer" author, one of my favourite short stories is Diamond Dogs, by Alastair Reynolds. For me, it's perfect sci-fi, it gets a re-read every couple of years..