Today I up-dated the Hollow Mountain project page 

'Hollow Mountain' a photographic documentary.

Deep beneath the surface, disused and abandoned mines are visually inspiring, dangerous, often difficult to get into and filled with the twisted, rusting remnants of our rich, mining past. They are historically, politically and geographically important.

Due to past mining activity, mountains once solid are now filled with enormous empty spaces; big enough to fit a sky scraper inside; Rhosydd Slate Mine, near Blaenau Ffestiniog, is one of several mountains in north Wales, which are literally hollow, just a shell remains, littered inside with piles of broken slate from collapsed mine passages and room sized chunks of slate that have fallen from the roof. The remains of crumbling passages and tunnels, railway tracks and trucks used to remove the slate remain scattered through miles of broken passageways. This mine, like many others, is geologically unstable and constantly collapsing. 

The beautiful Cae Coch sulphur mine, also in north Wales, was dug on a steep incline, it has crumbling decaying rock surfaces glowing with multi coloured bacteria, peppered with orange and red pools of sulphur where bacteria feed on the chemicals and minerals adding to it's rainbow of colours. Wooden beams supporting the roof are decaying, being eaten by bacteria and rotting from damp. 

The Dinas Rock silica mines in south Wales have miles of passageways with large rock pillars supporting the roof and passages filled with water. Again there is much evidence of past mining activity with caverns and tunnels that sometimes disappear into a deep void, while others close down and go nowhere.

http://www.h2ophotography.com/hollow-mountain