King Arthur's Labyrinth
While this is on a much larger scale, bits sound reminiscent of King Arthur's Labyrinth near Machynlleth. In the latter case the caverns were excavated as a slate mine, but still impressive - some of the main caverns are vast! Also complete with flooded tunnels (you enter by riding on a boat - with small outboard - through the flooded tunnels) and hard hats for the bits where the tunnel is about 4ft high. The aim of the tour is to view various bits of son-et-lumiere on the theme of the Welsh tales of King Arthur. Personally I'd pay just to explore the tunnels!
Well worth a visit if in the area. The adjoining Corris Craft Centre is home to Dyfi Distillery, makers of award-winning gin!