S'glhuo is a fictional dimension which first appeared in Ramsey Campbell's short story "The Plain of Sound".
S'glhuo is another dimension that only exists in our dimension as sound. In certain places, such as the plain in the Severn Valley, the dimension overlaps into our own, creating the effect of walking through jelly. Sounds from our world correspond to objects in S'glhuo, and sounds in our world can create things both living and non-living in S'glhuo. The location in the Severn Valley corresponds to a city of pillars on a planet composed of sound. It is implied there are other planets made of sound in S'glhuo and that the dimension is home to Tru'Nembra, an Outer God made from pure sound. In S'glhuo, our dimension is only perceptible as odor. Details on S'glhuo are given in the Revelations of Glaaki, along with a plans for a machine to contact the S'ghluoans.
It is not known if the blue skinned reptilians that live near the Severn Valley are the only type of S'glhoans, but it is likely that the dimension is home to other life forms. The denizens of S'glhuo may be friendly or hostile, but seem to have knowledge of both advanced science and the Mythos. In their own world they are blue skinned, tall, boneless reptiloids, but in ours they are beings of pure sound.