Deep underground, some of humanity's earliest artistic endeavors have lain untouched for millennia. The dark interiors of caves, wherever they may be found, seem to have had a powerful draw for ancient peoples, who littered the cave floors with objects they had made. Later, they adorned cave walls with sacred symbols and secret knowledge, from the very first abstract symbols and handprints to complex and vivid arrangements of animals and people. Often undisturbed for many tens of thousands of years, these were among the first visual symbols that humans shared with each other, though they...