The Black Ocean

When the Council of Eight rent the crust of the world and caused the oceans to empty into the Underdark, they unwittingly created The Black Ocean. An ocean in perpetual darkness populated with the remains of the demon army that once threatened the surface worlds.
The Black Ocean is a huge cave network that runs deep underground. Now that it is filled with the water from all the world's previous oceans, there exists almost no solid ground in the underground realm. Giant pillars of stone rise from deep beneath the water's surface to hold up the ceiling far above. The caverns are so huge that ships can pass through many parts of the Ocean, air currents providing some measure of mobility by sail. Many denizens of the deep prefer rowing to sails, as it is a much more reliable means of conveyance, if more labor-intensive.