The Lost City of the Jedi, a clandestine city and terraforming base, existed far beneath the surface of Yavin Prime's fourth moon. Millennia before the rise of the Galactic Empire, as the Great Sith War concluded, Jedi Order members pursued the rogue Exar Kun to Yavin 4. In an attempt to eliminate the Sith and end the conflict, the Jedi unleashed a devastating energy barrage upon the moon. This action successfully trapped Kun's spirit within his temple, but it also inflicted severe damage to Yavin 4's surface. This ecological disaster decimated Yavin 4's ecosystem, prompting the construction of the Lost City of the Jedi to revitalize the world.
Circa 3997 BBY, a rebellious Jedi apprentice, Exar Kun, allied with the Krath cult to establish the Brotherhood of the Sith, aiming to overthrow the Galactic Republic and the Jedi Order. Marka Ragnos's ghost had crowned Kun the new Dark Lord of the Sith, and he made his headquarters on Yavin Prime's fourth moon. The ensuing battles, collectively known as the Great Sith War, resulted in millions of deaths. Ulic Qel-Droma, a fallen Jedi and Kun's Sith apprentice, betrayed the Dark Lord, revealing the secrets of Kun's power. Consequently, a Jedi force, led by Qel-droma to Yavin 4, arrived to subdue Kun and bring the war to a close.
Unwavering, Kun chose to drain the life force of thousands of Massassi, so he could ensnare his own spirit within the walls of a temple. Determined to end the war, the Jedi joined forces and created a wall of light, a powerful burst of light side energy, which they directed at the planet. This potent, coordinated attack inadvertently ignited the moon's jungles, leading to the demise of the few remaining Massassi, demolishing most of the temples, and scorching the rain forests to the ground. Kun's spirit was confined within his own walls, but Yavin 4's ecosystem suffered greatly.
The Jedi Order endeavored to undo the damage caused by their wall of light and established an underground terraforming station to restore the moon to its prior state. In the aftermath of the fires, the moon's landscape had become desolate and frigid, but a group of Jedi Masters discovered they could alter the climate by devising a method to transfer heat from the moon's core to the surface. This involved drilling deep shafts into Yavin 4 designed to release both steam and heat into the atmosphere. This, in turn, led to the creation of the Weather and Climate Command Center. Once operational, the Center manipulated the moon's environmental conditions, making the moon warmer and more tropical. Following this, the Jedi initiated the seeding of the four continents, enabling lush forests to flourish again on Yavin 4.
Upon the completion of this terraforming undertaking, the Jedi began preparations for a subterranean city, which would eventually be known as the Lost City of the Jedi. The settlement was established on the moon's largest continent, constructed deep underground, beneath the rainforest, within a vast cavern. Employing deep shafts, similar to those used to release heat from the moon's core, they created tubular transports that facilitated travel between the underground city and "Topworld," the name given to the surface. One point of entry was a circular wall composed of green marble, situated in the forest. The underground cavern housed numerous dome-shaped dwellings for Jedi Knights, as well as various pieces of advanced equipment. Beyond its ecological significance, the new city also served to ensure that Kun's spirit would never be released.
Sometime after the city's founding, the organic inhabitants departed, and an army of droids assumed responsibility for the settlement's maintenance. The facility comprised travel tubes, computer cubes, and droids of every imaginable design. This high level of automation resulted in a bustling city, with droid occupants constantly modifying computers, creating new droids, repairing old ones, and performing other maintenance tasks, such as working on power generators, cleaning, and maintaining the hundreds of domes that dominated the city. Over time, the city faded from memory and was regarded as nothing more than a legend in the centuries that followed its creation. Nevertheless, the city remained active, and its droid occupants would occasionally venture to the moon's surface to gather information, which was used to update the history of the galaxy for the Lost City's Jedi Library.
Around 7 BBY, an unidentified brown-robed Jedi Master brought Ken, the alleged grandson of Emperor Palpatine, to the city for his protection. For twelve years, the boy grew up among the droids of the Lost City, shielded from the dangers of the dark side but unaware of his origins. After the Battle of Endor, the Bimm impersonator posing as the Supreme Prophet Kadann sought to acquire the ancient knowledge concealed within the Lost City. To locate the famed and forgotten city, the false Prophet abducted the young Prince Ken. He promised to reveal the secrets of the boy's origins in exchange for the correct geographical coordinates of the Lost City. Ken, troubled by this promise, provided the pseudo-Kadann with the information he sought. The Jedi secrets stored in the city's master computer nearly fell into the fake Prophet's hands, but the Grand Computer was destroyed by blaster fire, trapping Kadann and his minions inside the collapsing city. Although they eventually escaped, the city sustained enough damage that Exar Kun was now free to roam the entire moon.
The Lost City was constructed several miles beneath the surface, within a cavern illuminated by bubble lights and fluorescent rocks. The majority of the city's structures, once inhabited by the Jedi, were dome-shaped. The city was traversed by roads crafted from perfectly cut stone.
Within the Lost City of the Jedi was a vast library containing records that spanned thousands of years. The majority of the records were stored within a master computer, containing files placed there by the Jedi. In addition to these electronic records, there were also ancient manuscripts and books. This repository of knowledge contained information on all civilizations and the history of all planets and moons that harbored intelligent life.
This intricate facility was developed by the Jedi millennia ago and situated within the Lost City. It consisted of a network of pipe shafts that tapped into the moon's core, bringing heat and steam to the surface. This allowed for a degree of control over the moon's weather patterns, enabling its transformation from a barren wasteland into a tropical forest planet. Through a special code, the computer could be accessed and directed to release steam, creating new climate changes, such as advancing the rainy season.