On September 12, 1940, four boys and their dog, Robot, walk along a ridge covered with pine, oak and blackberry brambles. Robot begins digging near a hole beside a downed tree. The youngsters begin to dig, widening the hole, removing rocks—until they’ve made an opening large enough for each to slip through, one by one. They have discovered not merely another place, but another time. In the cool dark beneath the sunlit world above, the boys found themselves in “a Versailles of prehistory”—a vast series of caves, today collectively known as Lascaux , covered with wall paintings that, by some estimates, are close to 20,000 years old. In 1947, LIFE magazine’s Ralph Morse went to Lascaux, becoming the first professional photographer to document the breathtaking scenes. Now in his late-90s, Morse shared his memories of that time and place with LIFE.com, recalling what it was like to encounter the strikingly lifelike, gorgeous handiwork of a long-vanished ...