context: The Great Gun-Yu Flood, a story of a major flood related to the legend of Gun-Yu, [1] is a major flood event that lasted for at least two generations. This event caused massive population movements followed by other disasters, such as storms and famine. Residents leave their homes and live in hills and mountains, or build nests in trees. [2] According to mythological and historical sources, this event occurred around the 3rd millennium BC during the reign of Emperor Yao.
question: When was the first Great Gun-Yu Flood?
answers.text: 3rd millennium BC