“In the still, cool water of the dim tunnel, Alberto Nava scanned his surroundings as he ventured deeper into the maze of underground caves. Two of his friends tracked the guide wire that stretched along the passageway, leading the cave divers on and marking their path of return.

Eerie blue light from their lamps rippled across the limestone tunnel, picking out the protruding, reaching arms of ancient stalactites and stalagmites. Dodging the hazardous growths, Nava felt his heart rate begin to kick up. An hour into the dive, they were reaching the end of the line; the guide wire terminated just ahead, and beyond that lay unexplored depths.”