This dissertation theorizes an aesthetics of extraction in the modernist dialogue between U.S. and Mexican artists and critics in the 1930s. I analyze the ways in which perceptions of underground resources featured prominently in artistic theories about what was shared by U.S. and Mexican modernisms in the 1930s, as the United States renegotiated its access to the Mexican subsoil. As Mexico pushed to nationalize its underground resources, U.S. diplomats responded by stressing the transnational properties of minerals, mobilizing cultural diplomacy and the modernist capacity to transcend national borders. The artists and institutions I study reflect such a vision of the borderless underground: I argue that for each of them, minerals and the subsoil were conceptual mechanisms with which to produce expanded boundaries of American culture, challenging borders and the governing logics of flat, cartographic surfaces. To emphasize the apolitical, borderless quality of the subterrain, for instance, agents of the mineral frontier recruited authorities such as Diego Rivera and the Museum of Modern Art, who deployed aesthetic ideas about a formal “substratum” shared by Mexican and U.S. modern art. Primitivist dialogues by two lesser-known artists linked with Rivera, Jean Charlot and William Spratling, positioned Mesoamerican motifs as undeveloped “mines” for abstraction, at the same time that they conceptualized the materiality of minerals in their artwork as racialized reserves of dormant, primordial potential. And while some Mexican artists challenged the U.S. mineral frontier in canvases, murals, and political cartoons that explicitly championed Mexican control, others, like Rivera, did so by troubling ownership altogether, using muralism and cubist techniques to envision the subsoil as a collectivist, decentralized, and environmentally interconnected ecosystem.