Clay minerals are present in most sedimentary rocks. They find applicability in a wide range of disciplines, such as material, soil, earth, environmental, and biological sciences. Despite their abundance and use, swelling of clays under stress has not received enough scientific attention. We used a two-method approach, consisting of molecular simulation and nanoindentation measurements on montmorillonite. Our analyses of the molecular structure of montmorillonite at various stresses and hydration states showed that swelling behaves in a nonlinear way with stress. Nanoindentation results of Young's modulus agree with our simulation results, showing the importance of the interlayer in composite clay properties.