A 500-km wide region separating the East African and Ethiopian plateaus is one of the few places within the African Superswell with elevations below 1 km. The region encompasses much of southeastern Sudan and northern Kenya and experienced both Mesozoic and Cenozoic rifting. Crustal and uppermost mantle structure is investigated in the region by modeling Rayleigh wave dispersion measurements. Modeling results give Sn velocities of 4.1--4.3 km/s and average crustal thickness of 25 ¿ 5 km, some 10--15 km thinner than the crust beneath the East African and Ethiopian Plateaus. The isostatic response from 10 to 15 km of crustal thinning is sufficient to account for the low elevations between the plateaus.