What does Mozi mean by the "cave" in the siege war?

Cave mainly refers to the method of destroying the city wall through tunnel operation and opening up access for siege troops. Mozi called it "digging soil, binding columns and setting fire"; Shang Zi's Inner Chapter describes this siege method in detail. It says: "When it captured the city, the country was indifferent (calculating) the generosity of its city, and the country divided its land, attacking (fighting) with the ruler of the school (earthwork) ... The general rule is to accumulate wages, and the accumulated wages are to burn pillars. 18 people were trapped in the team (commando) (every broken wall gap) ... followed by the middle pawn (elite China army). The general is a wooden platform, supervised by the state and attended by Wang. * * * command observation siege warfare. In fact, the point attack is to hollow out the base of the city wall through the tunnel, support it with beams and columns, then set fire to the beams and columns to make the city wall collapse, and then command the commandos to seize the gap in the city wall and put their main force here to expand the results.