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Natural media like cold-land ice, vegetation, and dry sand are subject to a substantial penetration at microwave frequencies. For such media, the synthetic aperture radar (SAR) phase center is located below the surface, and consequently, the surface elevation determined with SAR interferometry (InSAR) is biased downward. For infinitely deep uniform volumes, the elevation bias is often equated with the penetration depth, but in this paper, it is shown that the two quantities generally differ. The interferometric bias is approximately equal to the two-way power-penetration depth if the latter is small compared to the ambiguity height, but for increasing penetration depth, the bias approaches one quarter of the ambiguity height. Consequently, no phase wrapping results even if the penetration depth exceeds the ambiguity height. The ratio of the InSAR elevation bias to the ambiguity height depends only on the ratio of the penetration depth to the ambiguity height, and the bias can be expressed in terms of the InSAR coherence magnitude, which makes it possible to correct the InSAR surface elevation for the bias. The volume depth can be considered infinite if it exceeds the penetration depth by a factor of two to five and if the surface scattering from the top and the bottom of the volume is negligible.