Basalt thicknesses in mare basins have been determined using assumptions about the premare topography of partly buried craters and by comparison to the Orientale basin. Differences in those assumptions have led to a factor of 4 difference in mare thickness estimates. Further, knowledge of thickness is restricted to areas in which buried craters are present. Using mixing models applied to multispectral images acquired by the Clementine spacecraft, we have shown that craters in the mare sometimes excavate highland material from below the mare cover. Using such craters and assumptions about their depth of excavation, we obtain independent estimates of basalt thickness. Our results are in agreement with the lowest of previous thickness estimates. We derive a volume of 40,000 km3 for basalts in Mare Humorum, considerably less than the ~110,000 km3 from previous estimates. We also confirm a diameter of 425 km for the Humorum multiring basin based on the Clementine gridded global topography, assuming an original morphology like Orientale. ¿ 1998 American Geophysical Union