The interesting thing is, at any point inside hollow earth away from the surface, gravity is immediately zero. Which means, given the imperfection on the inner surface of hollow earth, there are regions where gravity is zero at ground level and regions where gravity is higher than normal.
Given old records of kung-fu, in which pugilist are supposed to be able to soar through the air in a single bounce, Earth's civilization might have started in the inner world.
Qing gong indeed.
