Phlogiston

Phlogiston was a term that could either refer to the space between the crystal spheres of the Prime Material Plane or to the flammable fluid that permeated it, a rainbow-colored chaotic liquid that surrounded the spheres. Bizarre creatures inhabited it and preyed on spelljammers traveling between spheres.

Geography
There was a celestial configuration known as the Triad, which consisted of Realmspace, in which the world Toril was located; Greyspace, where the world of Oerth was found; and Krynnspace, home of the world of Krynn. The Phlogiston had a strong current that flew from Realmspace directly toward Greyspace, making travel in that direction relatively simple; due to this current, it was impossible to travel directly to Krynnspace from Realmspace. From Greyspace, however, it was a relatively simple matter to travel to either Realmspace or Krynnspace, strong currents leading to both spheres. From Krynnspace one could easily move to Realmspace, but travel directly to Greyspace from Krynnspace was impossible.