Three things come together here to make it fascinating

1   The tunnels themselves
2   The dereliction and the history it represents
3   The fact that it is forbidden territory

It's a bit like a shipwreck, you know it's there, you can get tantalising glimpses of it but it is the fact that it is inaccessible that preserves the sense of history in a way that bringing it to the surface and displaying it can't achieve