The story about him in Time at the time: https://web.archive.org/web/20080521163217/http://www.time.c...

    For Cray, the excavation project is more than a simple diversion. "I work when I'm at home," he recently told a visiting scientist. "I work for three hours, and then I get stumped, and I'm not making progress. So I quit, and I go and work in the tunnel. It takes me an hour or so to dig four inches and put in the 4-by-4s. Now, as you can see, I'm up in the Wisconsin woods, and there are elves in the woods. So when they see me leave, they come into my office and solve all the problems I'm having. Then I go back up and work some more."

That traditionally would be described as “a maze of twisty little passages, all alike”

"The unique complex contains more than 1,300 square feet of underground space, including tunnels and storage rooms - and even a hand-built railway."

(Edit: I mean the original article, not the submission)

that’s ~ 45 ft underground, I believe … perhaps not that impressive, but quite funny since I would hesitate to undermine my own house and surely this makes it worth less

You can get these types of things sorted (eg apply for retroactive planning permission) and the council will send a surveyor and give you an opinion but you are running the risk that they insist you either make it good (remedy anything they don't think was done right) or (worst case scenario) tell you to put it back like it was before the unpermitted work was done. This happens more often than people think.

That's why the house has been reduced in price so much - to find someone who feels like the headache is worth it. I have my doubts to be honest.

"Real" liability of the "if something happens I'm on the hook" kind is not what's driving down the value here. What's driving down the value is that now that there's a bunch of pictures floating around the internet the tunnels are worthless because some government inspector could choose on a whim to force you to fill them in at your expense (which you would presumably fight, expensively, in court).