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
The phlogiston was far from homogeneous. The fluid itself varied in its density from place to place and also displayed river-like flows, sometimes connecting objects such as the crystal spheres themselves within them. Spelljammers could make use of this flow to greatly increase their speeds through the vast distances within the phlogiston. Increasing the contact area with the flow to acquire greater speed was also the reason some spelljamming ships had sails.

The crystal spheres floated freely in the phlogiston, sometimes enough to significanty change their positions with respect to each other over time. The flow, however, remained unaltered, so navigation between the spheres remained possible. Many of these rivers flowed in both directions, but some flowed in only one way, forcing a return trip to take a different route.

In particular, there was a stable triangle in the flow consisting 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 flowed between Realmspace and Greyspace, making two-way travel between the two crystal spheres relatively simple. There were one-way flows from Greyspace to Krynnspace and from Krynnspace to Realmspace, but not in the opposite directions. Due to this current, it was impossible to travel directly from Realmspace to Krynnspace and from Krynnspace to Greyspace. From Greyspace, however, it was a relatively simple matter to travel to either Realmspace or Krynnspace, strong currents leading to both spheres. For that reason, traveling from Realmspace to Krynnspace was easier if one went through Greyspace first instead of going directly.

Another stable configuration, known as the Arcane Inner Flow, encompassed the crystal spheres of Refuge, a close neighbor of Realmspace and the site of a spaceport controlled by the Arcane, as well as Pirtelspace and the elven-controlled sphere of Darnannon.

Properties
The phlogiston was an opaque, brilliant, multicolored ether that did not resemble any of the four elements. It could not exist inside crystal spheres and quickly dissipated if brought inside, although it did permeate into a spelljammer's atmospheric envelope while it sailed through the phlogiston. It also could not be reproduced. Most importantly, it was an extremely flammable substance. Any spark or flame, magical or otherwise, immediately exploded, violently igniting the ship and its surrounding area. Because of this, travelers always extinguished their flames before leaving their crystal spheres. Enclosed areas of ships could only be illuminated by magical or flameless lights, such as those produced by fireflies.

Because of the obscuring nature of the phlogiston, crystal spheres were usually not visible during most of the duration of a trip. They only became visible at very close distances and, due to their enormous sizes, always appeared like perfectly flat walls with no apparent curvature.

The phlogiston also completely blocked access to other planes. It was impossible to access another plane while in the flow, even the extradimensional space created by a Mordenkainen's magnificent mansion spell or a magical item such as a portable hole. Except through the use of powerful magic such as a limited wish spell or similar, those regions remained inaccessible until the user returned to wildspace, which supported extraplanar travel normally.

Appearances

 * Into the Void
 * The Maelstrom's Eye