While roaming the Baltic Sea between Sweden and Finland in search of sunken treasures, the men aboard the Ocean Explorer came across something strange on their sonar: a cylindrical object they'd never seen before, 197 ft in diameter at a depth of approximately 275 ft. In a documentary filled with the tension and excitement of deep sea adventure, we follow the treasure hunters and their crew, through the lenses of advanced cameras and through the eyes of human divers, as they navigate threats both known and unknown on an expedition to uncover this extraordinary discovery