Researchers have made a groundbreaking discovery regarding the origins of non-meteoric water in natural spa waters located in central Japan. Based on numerical modeling, their results suggest that this water has been confined within the lithosphere for an extensive period of 1.5-5 million years. They identified three primary sources for this ancient water: the Philippine Sea Plate, the Pacific Plate, and ancient seafloor sediments, particularly in the Niigata and southwest Gunma regions.