Martin Butler's "Tunnel Vision" part 1

Conspiracy theories about secret tunnels under the military base on North Head are essentially the reason why this podcast exists in the first place, so we probably should have looked at the book (books?) that puts forward the best case for these conspiracy theories a long time ago. But we didn't - we're doing it now. This will be part one of at least two, depending on how far down the rabbit hole we follow Mr. Butler...