What is the origin of the slag hole?

Zhazidong concentration camp is called Zhazidong for short. \ r \ nIt is at the foot of Gele Mountain in Chongqing, 2.5km away from Baigong Pass. Zhazidong was originally a small coal mine in the suburbs of Chongqing, named after more slag and less coal. Zhazidong is surrounded by mountains on three sides and ditches on one side, and its location is relatively hidden. 1939, the Kuomintang military spy forced the mine owner to death and occupied the coal kiln, and set up a prison here. It is divided into two houses, the outer court is the spy office and torture room, the inner court has room 16, the first floor and the first floor are men's cells, and the two bungalows are women's cells. \ r \ n \ r \ nPrison structure: \ r \ n \ r \ nPrison: The concentration camp is surrounded by mountains on three sides, with a deep ditch in front. \ r \ n \ r \ nExterior: Outside the high wall, there are six sentries and a machine gun position, and a company of Kuomintang troops is often stationed. \ r \ n \ r \ nInner court: Zhazidong Detention Center is divided into two houses, with a discharge dam in the inner court and \ r \ n \ r \ nZhazidong concentration camp: 16 male prison and 2 female prison. Slogans such as "Youth is gone forever, think about it carefully", "Here and now, don't cling to it", "Endless maze, turn back to the shore" and "Patience with peace of mind, no regrets" are hung on the wall of the inner courtyard. \ r \ n \ r \ Outer court: The outer court is the office and torture room. There are slogans on the wall of the outer courtyard, such as "What the chief can't see, think, hear and do, we should see, think, listen and do for the chief". There are cells, interrogation rooms, warden rooms, interrogation tables, chains, bamboo sticks, pepper water, tiger benches and other torture. The "tiger bench" is a long stool, then hands and feet are tied to the stool and bricks are thrown at the ankles. When it fell to the third floor, its leg broke. Nearly 300 heroes were killed in Baigongguan and Zhazidong.