Built within the Temple Ziggurat, this vast chamber was destroyed and rebuilt several times in the Temple's long history, seeing completion in 1000 BBY under the Galactic Republic.
In 44 BBY, a string of robberies broke out in the Temple and the Jedi High Council assigned Jedi Masters Tahl and Qui-Gon Jinn to lead the investigation. After Jinn's quarters had been violated, the blind Master Tahl was able to recognize a foreign scent from Jinn's room as the smell of the lake. Summoning the Mon Calamari Padawan Bant Eerin to the lake, Jinn tasked her with searching the bottom of the massive pool for clues. Upon surfacing, Eerin brought with her a chest, similar to the ones used on her homeworld Dac. Opening the air-tight compartment, Eerin revealed the items that had been stolen from the Temple, including the lightsaber of former Padawan Obi-Wan Kenobi.
Jinn and Tahl returned to the lake soon after to set a trap for the mysterious thief that was vandalizing the Temple. Due to the heightened security, the lights in the lake level were powered off, allowing the two Masters to travel unseen throughout the chamber. Hoping to catch the thief upon his return, Jinn was about to capture a suspect when Tahl's Council-appointed navigation droid, TooJay, appeared and alerted the intruder, causing them to flee. While the trap was foiled, Jinn managed to get a glimpse of the thief and recognize him as Padawan Bruck Chun.
Upon his return to the Temple, former Padawan Kenobi walked with Eerin around the lake trying to think of a way to impress upon the Council his commitment to the Order and his sincerity behind the request to accept him back. As they walked the pair noticed that a horizontal turbolift car was dangling precariously from a broken shaft. Acting quickly, Kenobi accessed the service stairway that would take him up into the room's illumination bank. Climbing gently into the shattered shaft, Kenobi assisted Master Ali-Alann in the removal of the youngling he was escorting to the nearest dining hall. Meanwhile, Eerin had rushed to the High Council Tower, having found the turbolift control faulty, to notify the Council. The august body had rushed to the lake and used the Force to hold the car steady while the younglings were escorted to safety. Upon their return to ground level, the Council thanked Eerin for her quick thinking while they reprimanded Kenobi for acting alone. The Council deemed the lake level unsafe and closed it until repairs could be made.
Soon after Jinn and Tahl discovered that Dark Jedi Xanatos was behind the attacks, Eerin came to Jinn and told him about the tunnels running under the lake. Jinn, Kenobi, and Eerin entered the lake and dove deep into the water, accessing a hidden grate located in a boulder. Passing through the tunnel they were deposited into the main water service tunnel that provided water to the fountains and reflecting pools in that wing of the Temple. After searching the tunnels, Jinn confronted Xanatos over the water purification units; after a brief duel Xanatos slipped away and Jinn and the Padawans retreated back to the lake.
Several decades later, at the end of the violent Clone Wars, the Republic which the Jedi had served for so long was dissolved and transformed into the Galactic Empire. The Temple was placed under martial law by decree of Contingency Order 66. The 501st Legion of the Grand Army of the Republic, led by Darth Vader, stormed the Temple and executed its occupants, damaging the Temple in the process. The Temple's lake level was laid to waste, the water turned off, leaving the plants to die and the water to grow clouded and tepid.
The lake level would stand in silence for several more decades, slowly crumbling from disuse and lack of care. When Sith Lords Darth Sidious and Vader were slain, the Empire lost balance and was eventually driven from Coruscant by the New Republic. During the Yuuzhan Vong War Coruscant was devastated by terraforming, causing the Temple to crumble and reveal parts of the foundation. When the Galactic Alliance reclaimed the world, Chief of State Cal Omas, ordered the Reconstruction Authority to rebuild the ancient edifice as a gift for the great sacrifices made by the reformed Order against the Yuuzhan Vong. While the structure was rebuilt using blueprints of the old building, it is unknown if the lake level was reconstructed or replaced.
Located within the Jedi Temple, the lake level made up five stories of the massive structure. Upon entering it was said the atmosphere of the chamber made one feel as if they were standing on the surface of the planet. The floor was covered with grass and flowering plants. Trees were strategically placed near benches or rocks. At the center of the chamber was an expansive lake, the clear green water of which lapped the beach area with its gentle rippling. In between classes, it wasn't unusual for the younger apprentices to come to the lake for a relaxing swim in the calm waters. Lined with the stones and sand, the bottom of the lake concealed portals into the Temple's massive water tunnels which led away into the giant purification complex.
To continue the illusion of being outdoors, the ceiling of the lake level was covered in carefully placed light-banks that created the illusion of the outside sky. Beyond the illusion were catwalks and service stairways used to accommodate the Temple employees that maintained the light systems. Additionally, a horizontal turbolift ran above the lake connecting the dormitories with a dining hall.