Why is the Sea of Japan garnering the attention of oceanographers and nature lovers from around the world? Among other surprises, the Sea of Japan has its own distinct water mass, internal circulation, and an "activation device" that operates only in winter. Just 8000 years ago, the deep waters of the Sea of Japan were a "dead" zone. Now, they are a dynamic treasure trove of life. A 1000-m thick water mass hidden deep beneath its surface holds the key to this incredible transformation. How did this distinct water mass form? And how do the major changes occurring in the Sea of Japan foretell future environmental changes across the global ocean? A leading oceanographer with 40 years of field experience reports this oceanographic detective story to reveal the amazing secrets of the "Mother Sea.