Phlogiston

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.

Geography

The Flow's vastness is dominated by flowrivers, which link crystal spheres in a mind-bogglingly gigantic 3-D web. Flowrivers are useful to spacefarers because they (greatly) facilitate travel between the crystal shells. In many cases, a journey directly to a specific crystal shell would take many years, but the journey can easily be shortened to a much smaller amount of time by following the flowrivers from one shell to the next. (Hence, navigation in the phlogiston is a tricky connect-the-dots activity).
It is not possible to see a flowriver (though detailed measurements can detect a significant increase in the drift of the multi-colored fog in one or two directions). They are felt more than seen; an individual sitting on a spelljammer helm can easily realize that a starship has entered a flowriver.
Most flowrivers are two-way, allowing passage in either direction between two crystal shells. However, some are one-way, so it is not advisable to try to double back along a one-way flowriver (doing so poses the same difficulties as not using a flowriver at all).

The Flow is softly luminous, so light sources are not necessary on a ship's open decks while in the phlogiston. Visibility is quite limited, however - treat the phlogiston as heavy fog for that purpose.
Most places within the phlogiston look exactly alike, but there are known, and dreaded, Dark Regions from which few have returned. A feature unique to the Rainbow Ocean is the strange flow beacon, an object that resembles a rapidly spinning star. No one has ever determined who built these devices, nor what they do.

The phlogiston is extremely flammable within any kind of air, so the use of fire is highly inadvisable in the Flow. Even a spark can set off a small, fiery explosion. If a spell is cast that produces flame, it goes off instantly, centered on the caster, and it is considered to be cast as if on a plane with the minor fire-dominant trait.
For this reason, combat casters in the Rainbow Ocean tend toward the use of other energies besides fire, especially electrical.

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In the real world, phlogiston was a substance supposed by 18th-century chemists to exist in all combustible bodies, and to be released in combustion. Wikipedia: Phlogiston Theory