The floor of the tunnel was covered with breakdown-piles of broken bedrock fallen from the roof-but none were high enough to block her device. She pressed the button and the digital display showed 15.7 feet. She recorded it in her notebook.

The tunnel was three feet tall at its highest, not tall enough to stand in, so she would have to cross the space on her hands and knees. She eyed the breakdown. Not the most comfortable surface to crawl across, but easier than squirming on her belly. At least the knee pads would protect her knees from the sharp debris.

Diane shifted her backpack so that it hung from her side, stuffed the extra tether into one of the backpack pockets, and began the slow advance over the stacks of shaky rubble toward the opening in the wall ahead. She had gone perhaps half the distance when the floor beneath her seemed to shift a little differently with her weight. She glanced down at the angular rocks beneath her and hesitated, unsure what she was seeing.

Between the piles of rock was something strange-an optical illusion. She stopped and stared a long moment, moving her head one way, then another, studying the images. Looking through the space between several of the rocks was like looking through a View-Master-three-dimensional. She thought she glimpsed the tip of a stalagmite. With an electric flash of insight, she realized she was not seeing an illusion, but looking through to another chamber below her, and she was held up only by a jumble of rocks plugging a hole.

“Oh, shit,” she said out loud.

She moved a hand to her shoulder to turn on her walkie-talkie. The rocks shifted again.

“Mike,” she said, “I’m in trouble.”

“I’m not far behind you. What’s the problem?”

“I’m on top of some breakdown that has plugged an opening to a lower level. There’s no floor under me.”