Small-scale fluctuations of magnetic field and velocity may be responsible for the Stokes asymmetries observed in all photospheric magnetic structures (the microstructured magnetic atmosphere [MISMA] hypothesis). We support the hypothesis by showing that atmospheres with optically thin microstructure reproduce the polarization of Fe I λλ6301.5 and 6302.5 observed in a sunspot. Ten thousand spectra were fitted by model MISMAs with two magnetic components interleaved along the line of sight. Combining all the fits, we set up a semiempirical model sunspot characterized by two components with very different magnetic field inclinations. The major component, which contains most of the mass, is more vertical than the minor component. The field lines of the minor component are inclined below the horizontal plane throughout the penumbra. Magnetic field lines and mass flows are parallel; consequently, both upflows and downflows are present everywhere on the penumbra. Major and minor components have very different velocities (several hundred meters per second for the major component versus 10 km s-1 for the minor component), but the mass transported per unit time is similar. The similarity between the vertical mass flow and the magnetic flux of the two components suggests that field lines emerging as major components may return to the photosphere as minor components. If so, the observed magnetic field strength difference between components leads to a siphon flow whose magnitude and direction agree with the inferred Evershed flow. Several tests support the internal consistency of the retrieved model sunspot. The magnetic field vector B does not violate the ∇B=0 condition. The model sunspot reproduces the net circular polarization of the observed lines plus the abnormal behavior of Fe I λ15648. The use of only one magnetic component to interpret the spectra leads to inferring upflows in the inner penumbra and downflows in the outer penumbra, in agreement with previous findings.