From 18,000 to 10,000 B.C., long before Stonehenge, before the pyramids, before metalworking and before farming — back when mammoths and saber-toothed cats still roamed the Earth — prehistoric people painted deep inside these limestone caverns. [...] a “copy cave,” the impressive Lascaux II, allows visitors to see the reindeer, horses and bulls of Lascaux, painstakingly reproduced by top artists using the same dyes, tools and techniques their predecessors did 15,000 years ago. When I first heard a guide call the Lascaux caves the “Sistine Chapel of the prehistoric world,” I thought, “Promotional hyperbole.” Even if you’re not a connoisseur of Cro-Magnon culture, you’ll dig this cave — the last one in France with prehistoric multicolored (polychrome) paintings still open to the public. Font-de-Gaume contains 15,000-year-old paintings of 230 animals, including many red-and-black bison painted with a moving sensitivity — often in elegant motion. [...] the cave widens, sunlight pours in and you emerge — like lost creatures seeking daylight — into a lush canyon. The nearby Postojna caves are Slovenia’s single most popular tourist attraction. Postojna lacks Skocjan’s spectacular, massive-cavern finale, but the formations at Postojna are slightly more abundant, varied and colorful, with stalagmites and stalactites as tall as 100 feet. Here, an aquarium houses the strange, pale-pink, salamander-like “human fish,” a cave-dwelling creature that is celebrated as a sort of national mascot in Slovenia. Rick Steves writes European travel guidebooks and hosts travel shows on public television and public radio.