They walked along a track that wound through tall green bush. The sounds from the Outdoor Pursuits Centre faded away.
    
     'They reckon there are other caves up on those cliffs.' Sia nodded at where steep rock walls towered up on both sides. 'But they're too hard to get to and explore.'
 
     Todd gazed up through the ranks of cool grey trunks.  He imagined silent tunnels leading into darkness.
 
     Awesome!  He couldn't wait to get down into these secret places under the earth.
 
     After another quarter of an hour, they turned a corner, and saw a high wire fence.  Behind the fence, a dark opening led into a hillside.  The cave, Todd realised.
 
     Sia unlocked a gate in the fence, and they went through.  'Helmet lights on,' their instructor told them.  'Move carefully from now on.' As they entered the cave, the world of daylight and blue sky seemed to vanish for ever.  Their lights showed dripping rock walls where moss and a few ferns grew.  Ahead of them, the path sloped down steeply.