There's a bunch of potentially unpermitted tunnels under the property. Quite the liability.

And if they were permitted you'd still have the liability. It's not like the permitting authority is gonna say "aw well we let you do it this is on us" if something happens.

"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).