There’s a series of really unusual tunnels in Liverpool called the Williamson tunnels, after the guy who built them. They run under modern day houses and alleys, and a small section of the quite extensive network has been restored and turned into a heritage centre, here
There are no written records from Williamson describing the tunnels, so the motives for their construction are a mystery. Speculation has it that the guy was a benefactor to the poor, and simply paid local men to dig holes in the ground as a way of giving them work.
Whats really interesting about them is their architecture, very weird designs featuring unusual angles and archways. Parts of the tunnels are stacked on top of one another, so you have double and triple decker passageways. Definitely fires up the imagination as to what the guy was thinking about. Its a shame that only a small section is open for viewing at the moment, although the centre plans to excavate the entire network in the future.