For centuries, mankind has wondered what lies hidden in the depths of the Earth. For instance, what military secrets do underground bases like Area 51 and the Cheyenne Mountain Complex hold? And can discoveries in tunnels from the ancient past--like the Tayos Caves of Peru and the Longyou Caves of China--illuminate the cultures of lost civilizations? As humanity develops and digs further into the Earth, including the search for rare elements, what else will we uncover?