Archaeologists said on Monday they've discovered hidden remains of a prehistoric stone monument about a mile from Britain's famed Stonehenge site. According to researchers, at least 90 massive stone monoliths were found lining an "arena" along the southeastern edge of what later became the neolithic settlement the Durrington Walls. The stones, which archaeologists believe may have been used for religious or solstice rituals, were discovered lying on their sides beneath three-feet of earth and may have been placed there around 4,500 years ago. Vince Gaffney, one of the archaeologists leading the project, and his team unearthed the stone circle with ground-penetrating radar. Some of the stones are thought to have stood up to 15 feet high.