Hole excavating work to benefit sling passing is an essential procedure for wreck salvaging . at present the task is performed by divers with the mud penetrator , which is laboursome , inefficient and hazardous 攻打千斤洞是沉船打撈作業(yè)中的一項(xiàng)關(guān)鍵工序,即在沉船底部清理出一條穿引抬船用鋼纜的通道。