Pelaga is a series of four concentric spheres. In order from the center outward, they are the Core, the Inner Sea, the Skywall, and the Outer Sea.
The Inner Sea is a band of water, miles deep at its deepest and a few dozen feet at the shallowest, surrounding the rocky Core. It is populated by numerous lifeforms: fish and other aquatic animals similar to those on Earth, as well as massive creatures on par with the mythical kraken or the largest terrestrial whales. It is also home to the Folk, the label given to all intelligent beings.
Some Folk have fins or flippers. Others are humanoid. Many have something of both: a mixture of grasping arms with fully dextrous fingers, and finned tails. Like many of their unintelligent cousins, most of the Folk have a sonar or echolocation ability.
The Inner Sea plays home to dozens of cultures. Many Folk gather in cities, trading and entertaining each other. Others roam the ocean, acting as hunter-gatherers, craftspeople, information couriers, and so on. The Inner Sea has a bountiful harvest, but life is also hard at times. Hungry predators swim where they will to devour the unwary. Strange ocean currents can sweep incautious Folk hundreds of miles from home. And - worst of all - strange creatures fall from the Outer Sea, through the Skywall, to plague civilization.
The ocean has many currents. Some have been mapped out. These serve as natural trade routes: the Folk can enter the current, “go with the flow”, and emerge somewhere far distant, without expending the effort of swimming the whole way on one’s own power. Cities spring up around such places.
The whole ocean is illuminated by the “everlit”, or microscopic bioluminescent lifeforms. The Folk have been eating a diet containing everlit for so long that they, too, sometimes give off a glow of their own. Many other ocean lifeforms have useful properties that the Folk have taken advantage of. For example, walls and city structures are built out of long-lasting, durable coral.
Ocean water carries sounds better than air. The Allsong is more than just this: it is the pervading rhythm of the sea itself, a melody that all intelligent races can listen to and join in on. It is said that a new voice joins the Allsong when one of the Folk is born, and that their song can still be heard after one passes away.
Some Folk dedicate their lives to understanding and enhancing the Allsong. They serve as wise advisors, magicians, and communicators for their communities. A few unscrupulous singers understand that the Allsong is tied to the ocean and its inhabitants, and exhibit strange and curious powers.
The “floor” of the Inner Sea is the Core. Most of it is a sheer rocky face. However, the Core has numerous caves and tunnels within it. These channels are formed by sea-floor magma flows, and volcanic activity is still happening today.
The Core is a dangerous place to be. The largest and most ancient of the Inner Sea’s life forms make their home here in the eternal dark. Folk must bring their own everlit light sources to see here. However, the Core is the only source of minerals, gemstones, and other valuable materials. The rock of the core is sturdier by far than coral, and mined minerals can be refined into powerful weapons and armor.
At the top of the Inner Sea, floating “islands” bob on the surface of the water. The atmosphere is air, not water. Living things can be found here too. Birds, wheeling over the ocean and waiting to dive-bomb a fish. Four-legged herbivores or tiny insects occupy the lush, plant-filled islands. The Skywall is lit from above by clouds which give off their own light.
Most of the Folk don’t have much use for the Skywall. It’s the most convenient place to forge iron implements, though, so there are colonies or isolated settlements that have been established. Folk with legs can freely move around on the islands, while those with flippers or fins are usually out of luck.
Some of the Folk have organized themselves into a group called the Aeronauts. These brave (or foolhardy) explorers have an absurd goal: to find ways to lift themselves into the sky, and perhaps reach the Outer Sea. They use strange chemical concoctions which can fill "balloons" with a lifting gas. Soon, they hope, these “balloons” can carry Folk into the sky as easily as they already swim in the sea.
The Outer Sea is entirely unknown. Nobody has ever been there and returned. It’s not even known if the Allsong reaches to it, or whether the Outer Sea has its own Allsong.
What is known is that from time to time, monsters fall out of the Outer Sea, plummet through the Skywall, and land alive in the waters of the Inner Sea. These Leviathans are massive in size, and unbelievably hardy. Most have some kind of scales or other body armor. They follow no known body plan: they can mix fins, flippers, tentacles, and every other feature of Inner Sea life, or have weirder features yet. They frequently have more eyes than most creatures - sometimes many more, often running in rows along their tentacles or at the ends of eyestalks.
Leviathans are mindless colossi driven either by hunger, desperation, rage, or some less comprehensible motive. Nobody has ever communicated successfully with a Leviathan, whether with normal language, the Allsong, or other means. They are difficult to kill, and a whole community is often required to drive one away. Just as often, a Leviathan can decimate a sleepy hamlet of fishers or caravan of travelers, sending survivors swimming away in terror.
The Aeronauts have great hopes for their expeditions. If they can find the source of the Leviathans, perhaps they can stop the influx. If the Leviathans have a master, perhaps it can be reasoned with.
Player characters would be Folk from the Inner Sea. They would effectively be merfolk or other aquatic life, able to operate underwater and perhaps in the air.
Perhaps there is a strong Folk civilization that wants to take over the region. Perhaps the PCs must fight Leviathans - or learn their source, by joining the Aeronauts. The secrets of Pelaga might be found in some lightless cave in the Core, or beyond the Skywall in the forbidding and unknown Outer Sea.