Thousands of years ago, the world's most advanced civilisation sank beneath the waves, its people forever doomed to wander the ocean floor. The Underwater Realm is the story of the five times those on the surface have caught a glimpse below: freediving off the coast of modern Hawaii, a downed pilot at the height of World War II, amidst the smoke and fire of the Spanish Armada, a suicidal girl in medieval England, and finally, at the height of Roman Power in 149 BC.