Exonia

Exonia is a giant rocky planet with a vast hollow layer between its crust and core. It is inhabited both on its surface and within its inner cavity, and is a haven for adventurers since there is so much of it to explore, much of it untouched by "advanced" civilization.

Geology
Exonia's outer layer is roughly 6,300 km thick, consisting of a solid outer crust, then a molten layer, then a solid layer that serves as the ceiling of the inner regions. This cavity is about 1,200 km thick, full of a standard nitrogen and oxygen atmosphere, though at high enough pressures that most surface travelers require a mask or suit. This inner cavity also contains a number of large natural pillars of rock and iron that connect the outer and inner rocky masses. The core beneath has its own solid crust, liquid mantle, and magnetic core, with a radius of ~8,600 km, giving a total planetary radius of Exonia in the neighborhood of 16,100 km, over half that of Neptune or two and a half times that of Earth.

The outer layer of Exonia, while mostly made of tectonic plates floating on its mantle (which is itself flowing over the solid ceiling of the cavity), does contain numerous regions where non-molten columns extend all the way through the outer shell of Exonia. These regions are not moved (though still affected) by tectonic motion, and some of them contain cave systems easily surpassing all other known cave systems in the galaxy in terms of depth. There are three sinkholes that vertically and directly connect the outer and inner atmospheres, the largest of which is called the Abyss, and is roughly 10 km in diameter. Other cave systems likely have routes that connect the outer and inner atmospheres, but these are either unexplored or known only to local communities.

Ecosystem and Civilization
Exonia's outer surface contains all the environments typical of a tectonic, watery planet, including mountains, oceans, rivers, glaciers, plains, islands, etc. Roughly 35% of Exonia's surface is water, including polar ice caps. Much of the land is covered in various grasses, though there are sizable forests throughout the planet, consisting of various plant-, animal-, and fungus-type trees.

There are cities and other intelligent communities throughout the planet, though even the most sprawling of Exonia's civilizations have transportation networks that cover little more than three percent of the planet's outer surface area. This leaves much of Exonia wild, home to many rare, legendary, and even completely unknown species. Those that are documented include a vast variety of dragons, skybats related to those on Ventis II, the galaxy's largest Xillo Beast population near an ocean in the southern hemisphere, and a monstrous cephalopod that may be related to the Krakaken on Calpha.

Surface Civilization
The largest of Exonia's surface civilizations is the eponymous Exonians, a leathery-skinned pseudo-reptilian humanoid species who come in a variety of colors. They were the first on their planet to develop space travel, and the more empathetic of their nations shared that technology with other interested native species, such as the Giant Spiders. The largest Exonian society is in the northern hemisphere and boasts thousands of cities partitioned into six mostly-united nations, with trade routes and communication networks that span a respectable area, though on Exonia it is only about five percent of the total landmass. There are Exonians dispersed throughout the planet, though, and there are many communities that have almost no contact with any other members of their species. Some one-city societies are suspended over giant craters, some groups of villages spread across mountain ranges, some even retain a walled city-state setup where rival settlements dot the plains and are frequently doing battle. One interesting feature of Exonian culture is that virtually all have staff-based handheld tools. In the remote, tribal groups this means meticulously carved multi-tool walking sticks, and in the more advanced cities they range from powerful plasma generators for law enforcement to all-in-one personal devices that include keys, phone, digital wallet, lawn trimmers, grabbing extenders, and basic personal defense. Some practice genuine magic with their staffs, though this takes different forms depending on the culture. It is rare to see any Exonian without a staff unless they are either particularly panicked or particularly relaxed.

Another notable surface-dwelling intelligent species on Exonia is the Ku'untarg, a species of large, four-armed lizards with poor eyesight but notable strength. They are not a social species in that they usually live alone (unless they are raising young), but they maintain close relational networks with other Ku'untargs in the region and can usually find a friend if they need help with something. They are also often friendly towards other intelligent species and have a simple, respectful value system that even applies to their hunting. They usually hunt in what they call "the natural way" because they have little better to do with their time, though they have developed catapult weapons that launch explosive plasma pods that they use for in extenuating circumstances such as for defense, tackling large prey for (infrequent) group hunts, or as weapons on the very rare occasion that they unite for some battle. The plasma pods used as ammunition are harvested from a type of electric cave creature whose nature and location they keep closely guarded within their own species. While they are not technologically advanced beyond catapults, Ku'untargs are quite fond of gold and will be somewhat industrious and resourceful when it comes to acquiring it. Once they have it, they will wear it as ornamentation, mold it and play with it as a toy, or even chew it like gum with their powerful jaws.

Perhaps a more famous intelligent species on Exonia is the Giant Spiders. They mostly live in mountains and caves, and have complex social structures that incorporate a variety of species, from the meter-tall Shadow Spiders (black, quick, and hunt at night) to the gentle four-meter-tall Woolly Spiders (usually brown or grey, very furry, and the nurturing type). While almost certainly descended from "true" spider ancestors, their physiology has adapted so much to their larger size that they blur the line between arthropods and mammals, having both a fully developed warm-blooded cardio-respiratory system, and an exoskeleton. They construct their dwellings using mostly web and available materials such as wood and stone, but achieve city-like complexity of their dwellings, mostly concentrated in a single "urban" super-nest. Specialized hunters or trappers provide the greater part of the food for their villages, and will hunt just about anything that can't talk. One species, the Tree Spider, is entirely vegetarian and has little reason to live among other Giant Spiders. The Skeleton Spider is white and has a crab-like exoskeleton and cannot spin webs, and they are decidedly antisocial, living by themselves in open rocky areas. Non-arachnid species, notably the Exonians, long thought the Giant Spiders legendary, and even after their first contact, interactions were dominated by fear, since most humanoid species are generally afraid of spiders. However, some individuals ventured to make friendlier contact, and in the past century the Spiders and Exonians have come to a much better relationship, even to the point that there is a whole fleet of Exonian spaceships specifically designed to be piloted by Giant Spiders, though most Giant Spiders responsible enough to pilot are also not very adventurous, so these ships are rarely used except for emergencies.

Exonia is home to a vibrant dragon community, largely concentrated in a wide valley-plain called the Dragonwold. Dragons, of course, are extremely diverse, so they exhibit a range of social lifestyles, from the giant, gold-hoarding recluses to playful packs of dog-like dragons. Several dragons have been observed to have their own language, and there may be hope in translating it and conversing with them.

Finally, New Udok-Rif is a region of Exonia colonized by trolls, who arrived as refugees from their homeworld of Udok-Rif. These trolls show as much diversity in size and temperament as the dragons, but with a common language and what might be called a common culture. It is hard to describe their groupings in anything so concrete as "nations," since there is a great deal of cross-membership of clans between different larger gatherings. The one hard and fast division is between the East trolls and the West trolls, with the Eastern Trolls being the larger and more moderate group, and the Western Trolls being more industrial and aggressive.

Sub-Surface Life
Below the surface, there are a number of cities in Exonia's many cave systems, and undoubtedly many more societies based in yet-unknown caves. Some caves are known to be homes to dragons of several varieties. The Abyss is also home to several settlements. Some of Exonia's safer sinkholes are used as farms for a genetically engineered type of wheat that grows perpendicularly on the sides, and when harvested can just be chopped and dropped to the bottom of the hole for storage; these "wheat-wells" are common in more densely populated Exonian settlements.

Inside Exonia's cavity, life is a little more alien. Beneath inhabited caverns connected to the surface (such as the Abyss), scavengers can survive off of plant and animal matter that falls from above. Other than that, life has no connection to the sun's energy and must be chemosynthetic or pyrosynthetic. There are hot springs both on the ceiling and floor that support small ecosystems, and there are blimp-like ecosystems who feed on the rare but large amino acid clouds that are suspended in the dense air. The most impressive adapters are the pyrosynthesists, who use geothermal energy directly (instead of sunlight or chemical reactions) to drive the biological processes necessary to produce energy molecules like sugars. In order to use geothermal energy, creatures must be large enough to reach from the cooler air to the hot rock closer to magma. Since there is magma both in the inner core and in the crust above Exonia's hollow cavity, there are pyrosynthesists on both the ceiling and floor of this inner world. One such creature is the Macclar, only found on the lower floor and not on the rocky ceiling, whose "feet" extend hundreds of kilometers into the crust, creating a temperature gradient within its body that it uses to synthesize sugars; since these sugars are less stable at high temperatures, they are stored in tuber-like growths near the cooler surface. This makes it possible for other creatures to dig and find these "fruits" and thus Macclars have developed various defense mechanisms, but are nonetheless the foundation for an entire food chain. Above the ground, the Macclar has large, leafy appendages used for heat dissipation to maintain a steep temperature gradient throughout its body. These leaves are also attractive sources of meat that can't run away, but they are defended by razor-sharp scales, and by symbiotic creatures called Isk's Guards, which have a half-meter body and two 2-meter crablike legs. Isk's Guards are named for Isk, the first documented Macclar, itself named by a nearby tribe who also held it to be the first being in their creation myth.

The intelligent communities within this gaseous mantle are extreme adapters as well. There are a few hanging cities attached to the upper ceiling, though these were all established by outer-surface dwellers as sort of novelty (and tourist-attracting) constructions that also have served as bases for exploration of the inner world. Likewise, some self-sustaining blimps have been built to travel and explore the vast cavity, treated more like research stations with crew moving in and out on yearly schedules. On the inner core, there are native intelligent species, all of which have adapted for the substantial air pressures there (most outer-surface dwellers need pressure-regulated tanks and sometimes full-body suits to spend more than a few minutes even below the ceiling). Most intelligent life so far contacted on the core is tribal and not advanced beyond forging some metal tools. One group of warrior tribes is unique in that their expression of social status is the length of their mustache; they will shave the lips of any captives they have conquered in battle and some chieftains will even wear armor on their mustaches to avoid any accidents during battle.

Tourism and Exploration
Since much of Exonia is untouched by technological species, there is much to explore. Even with satellite imaging, the surface area is so great that it has not been entirely photographed with high-resolution equipment, and even then only a small fraction of that has ever been looked at. A vast majority of Exonia's forest rivers remain unmapped, and even supposedly large social communities remain little more than evident structures in satellite photos. This is to say nothing of Exonia's vast cave systems, and an entire planet in the interior that is impossible to see from above. As such, Exonia is known widely as the place where anyone can have an adventure. The presence of spacefaring civilizations on Exonia makes it accessible for any traveler, with many cities that boast all the comforts of a galactic community. These cities host a thriving business, then, of hosting travelers and finding regions of Exonia to explore that match the experience level of the traveler. At the current rate of adventure tourism, it is expected that Exonia's outer surface landmass will not be 99% explored for perhaps a century, again to say nothing of the caves and the inner cavity world.