The costs and complexity of the mine operation increase as extraction in an open pit mine progresses, making the use of underground methods more and more appealing. A natural question is, therefore, what are the boundaries of the open pit and underground sections of the mine that maximize economic value for the company? A few attempts have been made to answer this question, from sequential: open-pit first and then underground mining the remaining material, to complex simultaneous scheduling of both mines at different aggregation levels. The main focus of these works has been determining the optimal transition time. In this work, we focus more on the definition of the economic envelope of the mines rather than time. Our motivation for this is the robustness of this decision. For this we present a model and algorithm to jointly determine the optimal envelopes of the open pit and underground sections of the mine.11