Panthalassic worlds have oceans consisting of water at least 50km deep, no landmasses that reach the surface, but do have rocky cores. Waterworlds with oceans less than 50km deep are classified under the Bathypelagic subtype.
Many have a thick mantle of high-pressure ice beneath the ocean; the depth of the ocean depends on the gravity and temperature of the world. The atmosphere of this kind of world generally has a high water vapour content; if enough ultraviolet light is received from the local star(s) the water vapour will be split into hydrogen and oxygen, and on all but the largest panthallasic worlds the hydrogen will escape, leaving an oxygen-rich atmosphere behind. Other gases which may be present are nitogen, carbon dioxide (which may be largely dissolved into the ocean), argon, neon and (on the largest superterrestrial examples) helium.
During the early formative period, swift-growing gas giants often migrate inward through the protoplanetary disk. However, past a certain point, called the snowline, abundant icy materials used for gas giant growth become unavailable, and instead only rocky material of a lesser amount is used for planetary development. These worlds thus largely stop growing when they migrate inward, but remain composed primarily of icy materials. In the warmer region of a solar system, they then develop tremendously deep oceans and thick atmospheres.