Legend has it that in the Sanchalu area of Hongqiao District, there used to be a mass grave known as the "Ten Thousand People Pit." This pit served as a dumping ground for the corpses of Chinese forced laborers during the Japanese invasion. During the Cultural Revolution, a pair of young Red Guards visited this place to reflect on their experiences and enjoy some leisure time. They stayed late into the night and heard the sound of a wooden cart being pushed from underground. Some even claimed to have seen a cart carrying a heap of severed heads under the moonlight. The person pushing the cart was headless and appeared to exude a bloody substance. What could it be?