Beneath the Turkana Rift, which stretches across Kenya and Ethiopia, researchers say Earth’s crust has thinned much more than scientists realized. In places along the rift axis, it is only about 13 kilometers thick, down from more than 35 kilometers farther out. That degree of thinning places the region in a critical stage of continental breakup known as necking, when crust weakens, narrows and becomes more likely to split apart.