Experts have been puzzled by some ancient tracks left behind by sauropods, a group of gigantic dinosaurs which includes Brachiosaurus, Diplodocus, Apatosaurus and Brontosaurus. These creatures required four massive, tree trunk sized legs to support their enormous bodies, yet researchers have uncovered fossilized prints that seem to show sauropods walking on only two legs. There are two theories: 1) the prints were made in deep water and only two of a sauropod's four legs touched the bottom sediment, or 2) the firmness of the ground allowed only the creature's heavier front legs to leave impressions. The issue remains a paleontological mystery.