Chauvet-Pont D'Arc cave, in southern France, is one of the world's oldest and most impressive cave-art sites. Discovered in 1994 and popularized in the Werner Herzog documentary 'Cave of Forgotten Dreams', Chauvet contains hundreds of paintings that were made as early as 37,000 years ago. Fearsome animals such as woolly rhinoceroses, cave lions and bears dominate Chauvet's imagery. But one of its innermost galleries -- named after a giant deer species, Megaloceros, that is depicted there -- also contains a series of mysterious spray-shaped drawings, partly covered by the Megaloceros painting. A nearby gallery holds similar spray imagery, as does...