The Turks already mine gold 20% faster due to their civilization bonus and are also able to research Gold Shaft Mining. As a result, they have the fastest gold mining in the game. The Indians are the second fastest, receiving a 10% bonus from their unique technology Sultans.
When sources of gold and silver near the surface were exhausted, miners developed technology for going deep underground. This involved building lifts to send men down and bring ore up. It required pumps to pull water up and push air down. And it required new tunneling techniques for shoring up mine galleries and shafts to prevent cave-ins. Some of the richest silver mines were found in Eastern Europe.