To be accurate, the entrance of the underground palace was not discovered, but created by Cuisi's brute force.
This was originally a room to keep rubbish. It was obviously not created by the Goddess, but by the things that were needed by her disciples' basic such as eating and drinking. This included her disciples of all grades, including grade-12.
A small room within the temple was dedicated to the processing of rubbish. In the center of the room was a three-dimensional magic array. One only needed to throw the rubbish into the magic array and it would be disintegrated into its smallest atoms and categorized accordingly.
Saleen came to the room and saw the magic array that had been destroyed by Cuisi. Before giving him a chance to explode at him, Cuisi quickly said, "Master, this magic array almost killed me!"
