Swamp of Oblivion

The Swamp of Oblivion, also known as the Para-Elemental Plane of Ooze, was the Inner Plane at the intersection of the Planes of Earth and Water. It was a place of slime and mud.

Geography
The plane was a vast swamp populated by hags, enormous swarms of mosquitoes, and twisted trees. Few settlements existed in the place, built atop high stilts that kept constructions above the mud. However, since there was no solid ground underneath, everything eventually sank into the swamp.

Objects thrown into the swamp were rumored to disappear into the mud for at least a century. For that reason, powerful artifacts were occasionally thrown in the swamp by a someone desperate to remove it from the multiverse for a time.

At the border with the Elemental Plane of Earth, a hill range known as the Mud Hills were constantly eroded by the Swamp of Oblivion and regenerated by the Plane of Earth. The border with the Elemental Plane of Water was known as the Silt Flats, a muddy and sludgy region that marked the boundary between the swamp and the endless sea.