Lascaux.

On this date in 1940, two boys and their dog, Robot, found something ...

The story is so improbable, so marvelous, that it feels more like the remnant of a dream, or a half-remembered myth, rather than something that unfolded within living memory …

September 12, 1940. A warm afternoon in southwestern France. As two schoolboys hunt rabbits on a ridge covered with pine, oak and blackberry brambles, their dog excitedly chases a hare down a hole in the ground beside a downed tree. As boys will, the youngsters begin to dig, widening the hole, removing rocks, until they’re able to follow their hound down — and find themselves not merely in another world, but another time.

In the cool dark beneath the known world, the boys discover “a Versailles of prehistory” — a vast series of caves, today collectively known as Lascaux, covered with wall paintings 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, vibrant paintings.

Read and see more at LIFE.