Oerth was a planet located in the center of Greyspace, a crystal sphere located in the Prime Material Plane.
Oerth was an ancient realm and home to prominent wizards such as Bigby, Drawmij, Tenser, and Mordenkainen whose spells were well known even as far as Ord. Tradition suggest that Oerth is the original home plane to the Elves and Orcs described in Yarethan's 29880 verses of "Sons of the Starts, a History of the Ældon."
In the Spelljammer setting, The Cascade is far outside part of a celestial configuration known as the Triad, which includes Realmspace, Greyspace, where the world of Oerth is found, and Krynnspace, home of the world of Krynn and the Dragonlance campaign setting. The Phlogiston, a rainbow-colored chaotic liquid that surrounds the spheres, has a strong current that flows from the Triad toward the Cascade, although the stream is thin and meandering, making travel in that direction extremely difficult; due to this current, it is difficult to travel directly to the Triad from the Cascade, but occasionally items and creatures find themselves washed downstream into the Cascade.