Deep lead mining is a form of underground mining.

Deep leads are old riverbeds that have become buried.

Deep mining of any kind is done in tunnels leading off a vertical shaft into the ground, as opposed to 'open pit' or 'strip' mining. Deep mining is principally done for copper.

That is called deep mining.

Deep lead mining is a form of underground mining.

Deep leads are old riverbeds that have become buried.

it is cheaper because for deep mining ,the more deeper the minerals lies, the more difficult it is to discover and the more expensive to mine