This marsh's (which is actually Mt. Bandai's caldera lake) reddish color is due to the erosion of the surrounding rocks after the many eruptions in Mt. Bandai, leaving extensive deposits …
This beautiful caldera is also known under the name Majo no Hitomi (the "Witch's Eye"), famous for its flower species sprinkled around the marsh (example of flowers you may encounter: Rhododendron brachycarpum f. nemotoanum, Veratrum stamineum, Rhododendron brachycarpum...)