 In circa 1250 BC , the first wave of destruction has been witnessed in various centers of mainland Greece for reasons that cannot be identified by archaeologists . These incidents appear to have triggered the massive strengthening and expansion of the fortifications in various sites . In some cases , arrangements were also made for the creation of subterranean passages which led to underground cisterns . Nevertheless , none of these measures appear to have prevented the final destruction of the Mycenaean palace centers in the 12th century BC . The reasons that lead to the collapse of the Mycenaean culture have been hotly debated among scholars . The two most common theories are population movement and internal conflict . 
