The Material Planes and Phlogiston

There are many Material Planes, not just one. They all rest inside Crystal Spheres which dot the Phlogiston, a naturally-occurring magic flow which carries mystic powers easier than air, without interfering in any way.

Each Material Plane is itself infinite, and behave in unique ways inside their spheres. The plane most often used and referenced is Halordain, though there are infinitely many(And Halordain is a strange one at that, as it has both a Fragmented and Facetted Sphere, meaning that the angle in the Flow you approach through will determine where you appear on the plane proper, and there are many different spheres across the Phlogiston which all tie back to Halordain.)

The Phlogiston is considered coterminous with the Material Planes; That is, the two are interwoven so tightly and so evenly, with the Phlogiston supporting the Spheres of the many Material Planes, that they are considered the same singular Plane(Which is why you can't Plane Shift to another Material Plane, but also why you can't Teleport to another Material Plane.)