About White Jade Capital, there actually isn't much that needs to be explained.
Ling Yunpo merely described the scene and mechanisms once, and the three Kunlun Cultivators could immediately affirm that it was a secret trial ground for ancient human swordsmen.
As for why White Jade Capital "mysteriously vanished" during the Ancient Era, it can only be said that from ancient to ancient times, from ancient to modern times, too many things have been lost in history, and White Jade Capital is not a unique case.
However, it was the projections of the Sun Sword Master and the Tai Yin Sword Master that caused the three Kunlun Cultivators to ask several times over.
Upon learning that they were indeed only projections, they also sighed in disappointment, although it was expected.