Tirupati, Mar 10 A 3,500-year-old colossal mass burial ground, believed to be of the Megalithic period, has been unearthed by a research team of the city-based Sri Venkateswara University. The mass burial ground is spread over 100 acres with over a dozen ‘cist burials’, each measuring 30 feet by 27 feet, in Raakasi Mitta area, around 60 km from here, Professor at the Department of Ancient Indian History, Culture and Archaeology, Dr V Sakuntalamma, said.