1. A mining method, comprising: dividing a mining region into a plurality of federated mining regions; performing an open-pit mining operation in each of the federated mining regions and forming a pit having a slope in each of the federated mining regions, wherein the slope is formed into a stepped platform having a plurality of multistage platforms; performing an underground mining operation on the plurality of multistage platforms of the slope of the pit and forming a plurality of excavated tunnels; and backfilling a pit of a previous federated mining region with a spoil of a subsequent federated mining region; the backfilling comprising: processing the spoil into a paste; and filling the plurality of excavated tunnels with the paste.