About 12,000 years ago, hunter-gatherers who inhabited a swathe of Arabian desert carved life-sized images of camels and other animals on sandstone cliffs and boulders, using rock art to mark the location of water sources in an illustration of how ancient people tackled some of Earth's most inhospitable environs. Researchers said the monumental rock art was found south of the Nefud desert of northern Saudi Arabia at locales spanning a distance of about 20 miles (30 km) in mountainous terrain.