Deep in a cave in the forests of northern Spain are the remains of a gruesome massacre. The first clues came to light in 1994, when explorers came across a pair of what they thought were human jawbones in the cave, called El Sidrón. At first, the bones were believed to date to the Spanish Civil War. Back then, Republican fighters used the cave as a hide-out. The police discovered more bone fragments in El Sidrón, which they sent to forensic scientists, who determined that the bones did not belong to soldiers, or even to modern humans. They were the remains of Neanderthals who died 50,000 years ago.