Elysia

Elysia is a planet featured in Metroid Prime 3: Corruption. Its most notable characteristic is that the planet seems to be covered in a perpetual cloud of unknown gases. As a result, the only known inhabitants of Elysia have been the Chozo and the robotic Elysians who lived and maintained a large outpost named SkyTown high above the clouds of the planet, where the atmosphere is habitable (as Samus is able to survive there in her Zero Suit in the game's secret ending).

Although, it cannot be proven, the planet may be a Gas Giant as it is referenced that the crash of the Leviathan into the planet has struck its core, not its surface, which is consistent with gas giants being theorized to only having a dense core. Additionally, the planet's large size in comparison to other planets on the map and the violent lower atmosphere match characteristics of gas giants. Another possiblity is that Elysia may be a rock planet with a dense, violent and toxic atmosphere, as AU242 mentions how Skytown floats high above the surface, and shows a 3d representation of the planet, with the Leviathan sitting far away from the core. The large size of Elysia compared to the others in the Galaxy Map may be due to the interface of the map featuring depth, with that planet being the closest one to the screen. Complete Map of Elysia: 

The Birth of SkyTown
1500 years before the events in Metroid Prime 3: Corruption the Chozo built the outpost known as SkyTown, high in the skies of Elysia. The purpose for this outpost was to be an interstellar observatory, designed to launch many observation satellites toward distant worlds for the Chozo to study them. However, the constant winds and storms of the planet began to take their toll on the station. This took up much of the Chozo's time, and seeing the need for assistance in maintaining the station, the Chozo created the robotic Elysian race 50 years after the outpost's creation. The Elysians were given the gift of self-awareness to better assist the Chozo in this task.

The Discovery & Departure
The Chozo continued their observation of the cosmos, until one day, a Chozo Searcher came across a stellar object that was in many ways a planet, yet also apparently sentient. This chance event was brief, and contact with the satellite was lost before more information could be gleaned from it. Eventually, the Chozo departed the station, 400 years after the creation of the Elysians. With their departure, the Chozo requested that they watch over the station until others who came seeking knowledge and enlightenment arrived, as well as the request from the Chozo Searcher to continue exploring the skies for the answers to the mysterious sentient planet.

The Elysians continued the tasks entrusted to them, maintaining the station and continuing to search for the elusive planet. However, 200 years after the Chozo's departure, the Elysians were low on critical supplies and their attempts to produce fuel from Elysia's atmosphere was unable to produce a sufficient supply. As a result, the Elysians opted to enter a state of hibernation and have the data collected by the station fed into them by dreams.

Through these observations, many great events came to the Elysians, including the knowledge of the Chozo's new home on Tallon IV. The last of these dreams foretold of a meteor impacting upon the surface of Tallon IV, which brought an end to the Chozo civilization that was built there (setting in motion the events of Metroid Prime).

The Treaty of Elysia
1435 years after the Elysians' creation, the Galactic Federation made contact with the Elysians, when Federation forces activated SkyTown's automated defense systems. This revived the Elysians from their hibernation, which led to first contact with the Federation, and later the "Treaty of Elysia". The terms for the treaty were simple: the Federation would provide supplies, fuel and parts to the Elysians and in return, the Federation would be allowed to use SkyTown as they saw fit. This led to the installation of Aurora Unit 217 into SkyTown. This partnership benefited both sides as they both worked together, assisting each other in valuable research that would prevent many disasters throughout the universe.

Recent History
Five months before the events of Metroid Prime 3: Corruption, the Elysians detected a meteor-like object collide into a planet in a nearby galaxy. This planet may have been the Pirate Homeworld, although this cannot be confirmed. It is known that it was not planet Aether (whose impact was 5 decades ago according to GF data entries) or planet Bryyo (whose impact was at the same time as Elysia and the attempt on Norion).

Regardless, several months passed before the Elysians discovered that this meteor had come through a wormhole from an incredibly distant planet (which would later be revealed to be the planet Phaaze). It became clear that this was the sentient planet that the Chozo Searcher had been looking for. The Elysians attempted to transmit this information to the Federation, but were unsuccessful, as Aurora Unit 217 had become infected with a virus and thus communication with the Federation was disabled.

One week after the infection, the Elysians detected a Leviathan en route to their planet, destroying a large portion of SkyTown and ultimately striking the surface of Elysia. This was then followed by an attack by Space Pirate forces who proceeded to infect the Elysians with the same virus that had corrupted the Aurora Unit.

In response to this, the Galactic Federation dispatched the bounty hunter Ghor to SkyTown. However, Ghor too succumbed to Phazon corruption and as a result, took control of the Space Pirate forces, bringing an end to the Elysian resistance. This in turn prompted the Federation to dispatch Samus Aran to the planet, where she proceeded to defeat Ghor and the Space Pirates and destroy the Leviathan.

Biology
The only known native species to Elysia is the Sky Puffer, small airborne creatures who survive on the planet's abundant toxic gases. This is likely due to much of the lower atmosphere of the planet being inhospitable to life. The only known biological lifeforms who have lived upon Elysia for any extended period of time were the ancient Chozo and Aurora Unit 217, both with the aid of advanced technology. It should also be noted, however, that Federation and Space Pirate forces have had operations in SkyTown and some foreign species such as the Phazon Metroid have been able to survive within the confines of SkyTown.

Biological inhabitants aside, many robotic inhabitants have lived within SkyTown. For decades, the robotic Elysians who were made by Chozo have lived here with many other forms of robotic "life", manufactured by their Chozo progenitors or by the Elysians themselves. While there is no shortage of drone robots in SkyTown, it is presumed that the Tinbots, Steambots, Steamspiders, Steamlords, Defense Drones and even the machine Helios may represent the remaining, formerly self-aware members of the Elysian race. However this cannot be proven definitively, as during the course of the game they have been infected with a Phazon-based virus and cannot be reasoned with, even if these mechanoids were even sentient to begin with and not merely advanced drones. Indeed, the robotic Elysians who inhabited SkyTown for hundreds of years may have been wiped from the cosmos entirely, as the first and last Elysian lore entries imply.

Below is a full list of mechanoids and species that inhabit Elysia:
 * "Dragoon" Battle Drone
 * Advanced Aerotrooper (Space Pirate)
 * Aerial Repair Drone (Space Pirate)
 * Aeromine (Space Pirate)
 * Armored Pirate (Space Pirate)
 * Aurora Unit 217
 * Berserker Lord (Space Pirate)
 * Chozo
 * Crawltank
 * Cyrlic tree
 * Dark Samus (Appariation after fighting Ghor)
 * Databot
 * Defense Drone
 * Elysian
 * Elysian Shriekbat
 * Ghor
 * Helios
 * "Jolly Roger" Drone (Space Pirate)
 * Jumpmine (Space Pirate)
 * Leviathan
 * Parasite Queen (Space Pirate, deceased)
 * Parasitic fungus
 * Phaazoid
 * Phazon Metroid (Space Pirate)
 * Phazon Vine
 * Pirate Aerotrooper (Space Pirate)
 * Pirate Militia (Space Pirate)
 * Pirate Trooper (Space Pirate)
 * Red Phaazoid
 * Remorse-Class Turret (Space Pirate)
 * Repair Drone
 * Sky Puffer
 * Space Pirate ATC (Space Pirate)
 * Space Pirate Assault Skiff (Space Pirate)
 * Steambot
 * Steamlord
 * Steamspider
 * Swarmbot
 * Tinbot
 * Transportation Drone
 * UAM 4
 * UAM 5
 * UAM 6
 * ULF 12
 * ULF 14

Trivia

 * Although it is said that Elysia's atmosphere is inhospitable to humans, Samus is able to survive without her helmet and Power Suit as seen in the ending. This may be due to her Chozo infused blood that allowed her to survive on Zebes. This may imply that the Chozo (and Samus) can survive in environments that are too dangerous for normal humans, or possibly that higher altitudes might simply be more hospitable to organic life.
 * In Greek mythology, Elysium (Greek: Ἠλύσια πεδία) was a section of the Underworld (the spelling Elysium is a Latinization of the Greek word Elysion). The Elysian Fields, or the Elysian Plains, were supposed to be a Paradise that was the final resting place of the souls of the heroic and the virtuous.
 * This planet may be inspired by Bespin, a gas giant in Star Wars, with a city, floating in the sky.
 * In Corruption, this is the planet with the most boss fights.
 * There is a slight inconsistency in the SkyTown Data and Space Pirate Data entries. The former states that "a large portion of their home" had been destroyed by the Leviathan, but the latter states that "it missed the spy base". However, the Pirate lore may have meant that it missed the portion of Skytown that is specifically used for observing and collecting data from space for the Galactic Federation, while still having successfully hit the city, as mentioned in the Skytown lore; it can be assumed that the Elysians consider the entirety of Skytown as their home.
 * Since Elysia is a gas giant, and was plagued with the threat of becoming a Class XIII wasteland, the theorized eventual impact might have been more devastating than it was on other Leviathan-affected planets.

Areas

 * SkyTown, Elysia
 * Eastern SkyTown, Elysia
 * Elysia Seed

Major Upgrades

 * Boost Ball
 * Plasma Beam
 * Ship Grapple
 * Seeker Missile
 * Hyper Missile
 * Spider Ball

Bosses

 * Steamlord(3x)
 * Defense Drone
 * Ghor
 * Berserker Lord
 * Helios