The waters beneath Mahri Nui were known by many names, such as the black water and the deep deep. In fact, they were simply another part of the Pit. The escaped inhabitants of the Pit dwelt in these murky, dark waters. The black water connected the ruins of the original Pit to Mahri Nui above. Although one could reach Mahri Nui by traveling up through the black water from below, it was not recommended.
The ocean below Mahri Nui, known as the Black Water, was considered to be highly dangerous, inhabited as it was by the Barraki and their armies of sea creatures.
Sea water in some places was very corrosive.
There was a place within the universe that had corrosive water.