When archaeologists studying a fifth-century Swedish site found a human skeleton lying on the floor of a house, they wondered why no one had buried it. Then they found another. And another. All showed signs of having been executed with swords, axes, and clubs and then abandoned until the walls of their houses collapsed in on them. “It dawned on us that this was actually a massacre,” Clara Alfsdotter, a graduate student at Linnaeus University in Sweden and an archaeologist with the Bohusläns Museum, told the New York Times. They’ve found the remains of 26 people so far, along with a half-eaten fish (suggesting the attack was a surprise), coins, and jewelry, but they have not found any indication of who the attackers were or what they were mad about. Check out the ancient mysteries researchers still can’t explain.