Great Orme Bronze Age Copper mines.

The mines were originally found, by accident.  In that, the surface site was going to be made into a car park.  But they needed to do an underground survey.  That's when they discovered the mines.  And then decided to excavate the mines and open them to the public. Only the top two levels are, so far, available to the public.  The lower levels are being excavated.  But it's a slow process, to clear the passage ways and chambers of waste rock material.  Altogether there are 9 levels.   The mine was made, originally, by the people using stone and bone tools.  The passageways are kept in as natural a state, as possible.  The floor is sandstone.  And the passageway went uphill, before descending to the second level.  It also bent round several corners along the way :-).  

The photo is of a chamber in the mine.  Which is the oldest man made chamber.

It was an awesome experience.  Even with my vertigo threatening to kick in, at times :-)