Phlogiston

Phlogiston is the name given to the space between the crystal spheres of the Prime Material Plane. Bizarre creatures inhabit it and prey on spelljammers traveling between spheres.

In the Spelljammer setting, there is a celestial configuration known as the Triad, which consists of Realmspace, in which the world Toril is located; 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 Realmspace directly toward Greyspace, making travel in that direction relatively simple; due to this current, it is impossible to travel directly to Krynnspace from Realmspace. From Greyspace, however, it is a relatively simple matter to travel to either Realmspace or Krynnspace, strong currents leading to both spheres. From Krynnspace one can easily move to Realmspace, but travel directly to Greyspace from Krynnspace is impossible.