A painted bison on the ceiling of Altamira cave in Spain. The cave is closed to the public because human incursions have caused damage to the 14,000-year-old paintings.
A deer painted in Altamira Cave's "Polychrome Hall," a passage full of red ochre Paleolithic paintings. To the lower left are sensors to measure temperature and humidity in the cave.
Rocky Polychrome Hall in Altamira Cave has been compared to a Paleolithic Sistine Chapel because of its decorated ceiling.
Paleolithic cave paintings in Argentina. In Cueva de las Manos (Cave of the Hands) in Patagonia, people decorated cave walls with handprints at least 9,000 years ago.
The "blue caves" along the Greek island of Zakynthos are accessible only by sea. Sunlight reflecting off the water gives these caves their blue hue, making them popular for boating and diving.
Stalactites drip from the ceiling in a cave in the Dordogne region of France. These rock formations form when mineral-laden water drips through the cave, leaving behind calcium carbonate or other minerals. Water that falls onto the ground below from the growing stalactite can form stalagmites. When these ground formations meet up with the ceiling formations, the result is a "column."
The Kaklik cave in Turkey is fed by sulphur-filled waters, which create these white formations not unlike those seen at Mammoth Springs in Yellowstone National Park. The water's sulphur content gives these caves a stinky rotten-egg smell.
The Mulu Caves in Borneo are some of the most expansive on Earth. Carved out of limestone in Gunung Mulu National Park, the caves span at least 225 miles (362 kilometers) of underground passages.
A cave wall on the island of Antiparos, Greece. Cave formations such as stalactites and stalagmites are known by geologists as "speleothems."
Borgio Verezzi cavern in Italy was discovered by three young boys in 1933. It's now open for the public to view the stalactite and stalagmite formations inside.
Even today, humans can't help but jazz up caves. The Reed Flute caves in the Guangxi Province of China are illuminated with colored lights to highlight their formations.