<p>Due to the gradual increase in the depth of mineral resources, reserves, and open-pit mines, the use of simultaneous or non-simultaneous underground mining in continuation of an existing open-pit mine has gained attention. Therefore, determining the transition depth from open-pit to underground mining is significant. This research aims to determine the optimal transition depth using a scenario-based solution strategy based on the height of blocks in the block model, while the underground mining method, pit slope angle, product price, processing cost, smelting cost, and discount rate are considered variable parameters. This model determines the transition depth through a simplified and quick approach, which maximizes the total net present value (NPV) of copper and iron open-pit and underground mining. The main advantage of the presented model is that it considers all the technical and economic parameters of the open-pit and underground mining methods. The results demonstrate that block caving is the only underground mining method leading to a positive NPV in very deep deposits. In addition, the price of the final product has a significant impact on the transition depth. For instance, even supported or unsupported underground mining methods such as room and pillar and cut and fill can result in a positive NPV at high product prices. In this case, the transition depth is higher than the final depth of the optimum pit.</p>