We have examined the amplitude ratio SKKS/SKS from two deep focus events recorded by the APT89 portable seismic experiment. For one of the events (beneath the Solomon Islands), we obtain a profile of 18 stations in the distance range 99--108¿. We observe large systematic variations in this ratio as a function of distance. From 99¿ to 106¿ the ratio is fairly constant and in the range 0.1 to 0.5, but it then rapidly increases to the range 0.5--3.5 between 106¿ and 108¿. Such variations are in fact predicted by standard earth models, such as PREM, although this feature of the wavefield has not previously been observed. It is due to the momentary drop in SKS amplitude at the ray parameter where the reflected phase ScP is critically reflected. Because ray theory is not valid in this distance range, we use Full Wave theory to model the amplitudes. We have been able to match the distance pattern reasonably well if we use a model with the P-velocity at the base of the mantle reduced by 1.5% compared to PREM. Although fewer stations are available, the second event shows the same basic distance pattern, except that the abrupt increase in the ratio occurs at a closer distance. This suggests an even lower P-velocity for this region (about 3.5% slower than PREM) and is consistent with lateral heterogeneity at the base of the mantle of a few percent. ¿ American Geophysical Union 1993