Before revision: In mining, they were used to drill into the soft carbonate rocks of lead and zinc hosted regions to extract bulk samples of the ore. History Churn drills date back to as early as Qin Dynasty China .
After revision: In mining, they were used to drill into the soft carbonate rocks of lead and zinc hosted regions to extract bulk samples of the ore. History Churn drills were invented221 BC in, capable of reaching a depth of 1500 m as early as Qin Dynasty China .