Excerpt from The All-Seeing Eye, my young adult science fiction/fantasy novel:

"Ron stopped, sucking in the stale air that was becoming harder and harder to breathe.  At first he thought that his travels had been in vain, for there seemed to be nothing there.  Had he made a wrong turn somewhere?  But at that moment he noticed a strange stone fastened to the wall on one side of the passageway.  Carefully Ron examined it with his flashlight, but the stone seemed not to belong in the cave at all.  It was extraordinary with an eye in the center surrounded by bizarre patterns that came alive as Ron touched them.  He was completely unprepared for what happened when he turned the strange stone.  He leapt backwards in alarm as a doorway appeared and the darkness of the cave was illuminated instead by the brightness of a billion suns.  Even more astonishing was the great rushing noise that came from the door.  It sounded like hundreds of thousands of gallons of raging water, tumbling over a waterfall in a thunderous roar.  Ron’s senses felt like they might explode as he turned the stone once again.  The door closed as suddenly as it had opened.  He stood against the wall; his chest heaving, his blood racing, his mind alive with wonder.  It didn’t take him long, however, to steel himself against what he had to do.  He knew that the answers to everything that he ever wanted to know lay on the other side of that door.  Whatever peril lay ahead, whatever there was on the other side, Ron just had to know."