About twenty kilometres south of Iringa, on the Mbeya road, Isimila is two things at once: an open-air Stone Age site where hand axes and cleavers, made by Homo erectus or early Homo sapiens, still lie on the surface of the ground; and a small canyon of strange eroded sandstone pillars that look like a set from another planet. A small site museum keeps a few of the better tools and bones, including a fossilised hippo skull, and a guide will walk you down into the gully.
The pillars are reached on foot from the small visitor centre on the main road; the walk is gentle but exposed and very hot at midday. Bring water and a hat. Entry is around US$10 for foreigners; pay the official rate at the gate.