A look at what could cause once-thriving groups of people to just abandon their great cities, never to return; modern scientific knowledge may be able to help answer the unsolved questions surrounding these lost civilizations.

Amir Hussain, Andrew Collins, Carl Wendt, David Whitehead, Dr. Travis Taylor, Edwin 'Ed' Barnhart, John Brandenburg, Jonathan Young, Paul Bahn, Paul Springer, Rob Weiner, Tok Thompson