Many see them as massive testaments to America's progress. Some view them as immutable symbols of the human spirit. Others perceive them as sacred shrines dedicated to the ideals of liberty and freedom. But no matter what they represent--America's monuments are inarguably impressive due to their colossal size and scope, their groundbreaking feats of engineering, and their storied histories. Why is it that America's greatest monuments continue to enthrall us decades--and sometimes centuries--after their creation? And perhaps more importantly, what closely guarded secrets may their implacable facades be hiding?

According to the Greek philosopher Plato, the lost continent of Atlantis was once home to an advanced civilization before it was destroyed in a cataclysm. But was Plato's story merely a myth? Or did Atlantis really exist? For centuries, archaeologists have tried to uncover evidence to prove that this legend is true. Will Atlantis finally be re-discovered, perhaps by employing modern technology? Or will this lost civilization remain...UnXplained?