the ancient maya created magnificent city-states. here three million people once lived. in the earliest cradle of civilization, ancient mesopotamian farmers once made these deserts bloom. halfway around the world, in california, are clues to understanding the fall of mesopotamia, as farmers here struggle to overcome a threat to this fertile garden land. the ruins of ancient societies may hold keys to our own survival as, out of the past, archaeologists explore one of the greatest of mysteries the decline and fall of grand civilizations. mission control: ignition. and liftoff. liftoff. keach: for more than five millennia, humankind has seemed to dominate earth, both creating and destroying grand civilizations. each of these human experiments has changed our planet. this high vantage point brings us a new and sobering view. for the first time, we behold our world as finite, limited. on the darkened face of earth, the lights of cities record the expansion of our kind. just 50