The Core is a pocket dimension created by Gaia located in the center of the Earth. Only kiji can enter, although Gaia has the ability to lock them out. Currently only Gaia and Kokoro permanently reside there. Gaia does not have the ability currently to leave the core, although she can see what goes on in the world through the senses of the kiji and nature itself.

The Core is a massive, singular size, unchanging in dimensions. It is approximately 760 miles (or 1220 kilometers) in diameter, meaning it takes up all the space of the Earth's real inner core, but on a different plane. It is impossible for anyone to dig straight to the Core, due to Earth's outer core functioning as a super toasty (read: lethal) natural barrier. Kiji, however, are able to freely enter and exit the Core, thanks to multiple portal routes littered all over the world.

Despite the Core technically being the center of the planet, it is not inhospitably hot thanks to Gaia's power. If a human for example entered, they would not be incinerated, although painful sunburn is likely! The Core is centered in the middle of the planet because it is the most viable location to be able to spread energy throughout the world to help keep it stable.

The Core also has a unique way of dealing with gravity. There is a false, miniature sun-like orb in the center of this hollow dimensional sphere, and the walls of this sphere act as the floors, gravity behaving in an inverted manner. 

There is vegetation, although somewhat sparse. None of it is made from the planet; all of it is created from pure energy, made to reside within the Core. That means despite it looking like there's plantlife, this is a semi-solid illusion created by raw energy. Should any living creature try to make a home here, they would die of starvation.

There is naturally no wind, unless generated by the kiji themselves or by Gaia’s presence.

Minor Points of Interest

Kokoro's den area, up near the central sun core. Although they don’t live inside the false sun, they’re so close that to most kiji, they might as well. Kokoro’s den is largely rocky, and there’s some false plant life to help break up the monotony. There is a small, private alcove where they like to rest. Where Kokoro often likes to lounge is a raised boulder, a bit higher than the rest of their abode, and many kiji refer to this as Kokoro’s ‘throne,’ though it is not shaped like one.

Three small 'village' type locales, inhabited by visiting kiji who are coming to rest after depositing energy. They recuperate and socialize here. These ‘villages’ are cleared areas made up of pits, alcoves, small caves, tents, or den-like structures. One village has something resembling an arena, where spectators can watch other kiji “resolve their differences.” It’s almost never a serious fight that occurs in this arena; oftentimes it’s kiji comparing their skills or features and bragging about themselves.

There's two pools of water that kiji often frequent for summoning purposes, making them hotspots. Larger numbers of kiji tend to gather at these pools for finding other summoners or if they need advice from other kiji on how to summon. Those who like to summon frequently also find themselves at either of these pools.

There's a cave where kiji seclude themselves often, for rebirthing in relative privacy. As rebirthing isn’t being done constantly by everyone, while there’s a bit of a queue, generally privacy is guaranteed.

As for Gaia's preferred abode, she doesn't technically have one. Due to how she built the Core, and the events leading up to and during the Thinning, she had such little energy that she ended up becoming the Core itself. She does not have a physical, visual appearance, save for the environment itself. Under rare circumstances, she can take on a brief humanoid manifestation when called upon by Kokoro or other valued kiji.

The location of the entrance/exit of The Abyss, is currently located in the Challenger Deep within the Mariana Trench. The two big reasons for this are because it's safe to hide in, secure from kiji and other predators. And two, it's as close as Yile can get to the Core without being noticed by Gaia.

Unlike the core which has a physical presence, Yile's Abyss is much, much larger than the Core, but has no physical presence on the planet. Once the portal is accessed, one is greeted with the Abyss. It's so large that generally the Subtalleons congregate near the entry portal. If one goes in too deep, it's extremely easy to get lost, and one is unlikely to see many friendly faces.

Inverse to how Kiji treat the Core, Subtalleons try to avoid the Abyss unless it’s to speak to Yile, hide from Gaia or Kiji, or any other threats. Meld will also take unhatched Subtalleon eggs into the Abyss to keep them safe, if needs must, but eggs cannot hatch there. Subtalleons that have recently hatched will also enter the Abyss, just for safety in their early existences.

The environment is ice cold, and should a normal mortal enter and remain longer than a minute, there's a high chance for frostbite, and an impending sense of doom typically sets in. Hypothermia can set in within around ten minutes for an average, living creature. Subtalleons, of course, are not impacted by it at all. Kiji entering would not die of the cold, but rather the Abyss would drain their energy completely, as with any other creature aside from the Subtalleons.

There is a constant current here, almost like a mix of wind and water, one with a visible presence. It goes largely unfelt, but one can see the flow of the currents if they look closely. The Abyss also can skew one’s sense of time, making it impossible to tell how long one had really been there. If they were, in reality, inside for ten minutes, when they leave, they could discover only five minutes or fifty minutes had passed instead. Even Subtalleons don’t trust their sense of time in the Abyss.

The reason it's so empty is mostly because of Yile's experience in the deep void of space, so she doesn't understand the idea of decorating very well. She's also fairly selfish, and doesn't care about what others might think of her lack of hospitality.

Minor Points of Interest

Unlike Gaia, Yile does have a physical presence, and a black, smooth obsidian-like throne she likes to sit on. She does wander the Abyss, though she can usually be found around her throne. Aside from the main portal, this is the only real point of interest. Aside from the Abyss being vast, there is very little in it that takes up space besides the fanciful clouds, false stars, and water reflection effects.

Another point of interest is Meld’s Nest, which is an extensive area, roughly around 5000 square feet (or 465 square meters), that’s filled with varying types of eggs that hold Subtalleons, all in different stages of incubation. Most Subtalleons will drop off their eggs here and let Meld care for them, meaning most Subtalleons never know who their personal summoners are unless one comes to claim responsibility. The eggs are usually hidden and protected by rocks, kelp, and sand, but not buried or concealed in holes. Due to the size of the Nest, Meld frequently has volunteers who come and go to help tend the eggs.

There is also Stark and Trifle’s lab holed up in the Abyss, far from Yile’s throne, where they spend their infinite time creating strange gadgets and trinkets, as well as researching the world and its many wonders and oddities. Yile does visit fairly often, and Subtalleons will come to them sometimes for information.

Vault's Haven is located on the outskirts of a town. It's a fairly large property, surrounded by a picket fence. It's lush with trees, bushes and flowers, and a creek runs by in front of her home. She does tend the flower field behind her home, as well as her gardens out front where she also grows fruits and vegetables. She harvests these for her own cooking, or sells the surplus to townsfolk. Though the majority of the ground on her property is grass, there is also a vast amount of clover, and the path to the home and around to the back is packed dirt and sand. Leading beyond the picket fence and to the town is a gravel pathway.

Her cottage itself is an average size, being built for three people, two stories high, and is made from a mixture of wood, clay, and stone. On the side of her home is a chimney and a covered area for firewood. She does not chop the wood herself, but buys it in town and brings it home. 

The inside of her home is spacious and minimalistic, with a plethora of potted plants and herbs, and small decorative trinkets. Self-woven tapestries decorate the walls, and soft throws litter the furniture. On the ground floor is a combined kitchen and dining area, a restroom, and on the other end is the living room, where the fireplace is located. The stairs leading up are located at the far end, which open into her bedroom, a guest room, and a small room with a balcony in the back for Keep. There is a bath built out back in a small shelter.Out back, there is a bath built in a small shelter. Additionally, a little further from her house, is a medium-sized, divided coop for chickens and rabbits.

Kiji frequent Vault's Haven, so the place is often rife with them, but the only one who really sticks around is Keep. Due to the inviting nature of Vault’s property and Vault herself, local wildlife also visit often, consisting mainly of squirrels, raccoons, possums, and deer. She’s most certainly not a princess from a particular franchise.