Crystal Cave.

If you think that caves are dark and creepy, take a look at the one geologist (rock scientist) Javier Garcia-Guinea stumbled on last June in Almeria, Spain. His find? A gigantic cave lined with perfectly clear crystals. The cave, known as a geode (hollow crystal-lined rock), is the largest ever found in the world.

While most geodes are no bigger than your fist, Garcia-Guinea's crystal hideout can fit you and a handful of friends comfortably. Its cavernous center measures 8 meters (26 feet) long and 1.7 m (5.6 ft) high, and is lined with towering crystals, some up to 2 m (6.6 ft) long. The crystals--made of the mineral gypsum--are so transparent you can read a newspaper through them!

Geologists think the gigantic geode was formed 6 million years ago, when mineral-saturated water flowed through a rock hole. As water slowly evaporated, minerals clung to the rock's surface, forming a lattice of smooth-faced, geometrical shapes. "When minerals separate from water they stick to solids, just like sugar from a solution sticks to a piece of string," says Paul Burger, a geologist at New Mexico's Carlsbad Caverns. "Crystal formation uses the same process that makes rock candy."

Unlike rock candy, however, these crystals may hold vital clues to the region's ancient climate, water, soil composition, and life forms. But Garcia-Guinea has more modern plans for the cool, sparkling cave--he hopes to convert it into a hot tourist site.