The underground hall is also called the patio. "Four eaves dripping into the atrium" is an old saying of folk house science, which means that money at home does not leak or accumulate. Therefore, the atrium of ancient dwellings is hollow, commonly known as "patio". The eaves of the two veranda rooms and the central hall incline inward to the patio, so that rainwater can flow back to the patio. It also plays the role of circulating air. (There were no air conditioners and fans in ancient times)
Because it is empty, the moonlight can shine in. On a hot summer night, it is really a pleasure for the family to enjoy the cool in the patio (underground hall).
My humble opinion, please correct me.