Indentation is one of the simplest ways to feel the stiffness of matter. Interpreting indentation, however, requires care, especially for thin films—such as biological cells or soft coatings supported by rigid substrates—because the substrate effect can make a film appear orders of magnitude stiffer than it actually is. To minimize this artifact, the empirical “10% rule of thumb,” which limits indentation depth to one-tenth of the film thickness, has often been adopted, though it becomes unreliable and impractical for ultrathin films. Here, we explore the traditionally forbidden regime: deep indentation, where the indentation depth is comparable to the film thickness. We develop an asymptotic theory that incorporates nonlinear geometry and elasticity, elucidating a simple indentation force-depth relation in this deep regime. We further show that the pull-off force in such indentation tests is remarkably independent of both the material law and geometric nonlinearity. The results provide a unified framework for interpreting thin-film mechanics under deep indentation, a regime that is generally much more accessible in experiments on very thin films.

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• Continuum mechanics
• Elastic deformation
• Thin films
• Indentation