Star Wars tie-ins have revealed the Jedi Temple's greatest secret, but that secret has yet to be explored in any real detail. Little by little, Star Wars is revealing the history of the Jedi Order. Luke Skywalker discovered the first Jedi Temple on the remote world of Ahch-To, confirming the Jedi developed far from the Galactic Core. Early Jedi explorers used the Force to travel the galaxy, trusting it to guide them to worlds rich in life. The Jedi apparently settled on Jedha for a time, claiming it as their own world.
The Jedi's relationship with the Republic changed and developed over the centuries, though, and the most important Jedi Temple became the one found on Coruscant. Coruscant's Jedi Temple was built on an ancient Sith shrine, a nexus of dark side power that the Jedi hoped would be contained by their light. Claudia Gray's novel Into the Dark revealed that Jedi of the High Republic Era - centuries before the prequels - actually used this dark side vergence. They would take relics and artifacts to it, using the Force to strip them of the dark side and deposit their power within the vergence. They believed it would then dissipate into the cosmic force.
The Jedi Used Coruscant's Force Vergence For Other Purposes
Star Wars #20, by Charles Soule and Marco Castiello, reveals the Jedi used it for other purposes. The issue sees Luke unlock a holocron recording left by Master Yoda, and his mentor gives him a brief description of Force vergences. "Locations exist where the Force is more sharply focused," Yoda observes, "but dangerous these places can be. Vergences, they are called, and should not be explored without proper training and preparation. The Jedi Temple on Coruscant has one such. The Living Sea of Gazian, another. Jedha, as well." According to Yoda, these three planets are rich in the Force, and can be used to peer through the mists of time and space.
Star Wars #20 explores the Living Sea of Gazian, revealing Force-sensitives can immerse themselves within its waters to receive visions of the past and the future. George Mann's recent audiobook The Battle of Jedha has revealed the Force vergence at Jedha, a temple filled with mirrors of kyber; a Jedi would meditate in the presence of these mirrors, receiving visions from the Force. Interestingly, the descriptions given in The Battle of Jedha are reminiscent of similar visions experienced by Darth Vader when he meditated on his fortress on Mustafar, so it's possible there are other vergences that can be used in this way - ones the Jedi didn't know about.
But the Jedi Temple's Force vergence has seldom been seen, and there have been no accounts of it being used in this way. It's possible the Jedi had discouraged its use by the time of the prequels; Yoda in particular had come to believe Jedi were unwise to seek visions, believing they should instead be allowed to come naturally. That would certainly explain why Star Wars has yet to show this aspect of the Jedi Temple on Coruscant.