The Sof Omar caves are one of Ethiopia's secrets. They are reputedly the largest cave system in Africa with up to 16km of tunnels having been explored which have been carved out of the limestone rock by the Web River. They lie about 100km east of Bale and can be reached from Goba and Robe by 3 hours of rough driving on a disintegrating cobble paved road. The cave system can be entered at the Holuca Resurgence through the Chamber of Columns, a hall studded with limestone pillars. There is a fairly easy walking trail which leads for over 1km into the caves which the local boys with torches will happily lead you along - for a fee of course. The caves are named after Sheik Sof Omar a 12th century Muslim leader who used them as a refuge. The Web River is an important source of water for the local area and by mid morning the valley above the cave is a congested traffic jam of herds of animals being led up and down for watering.
For more information see http://en.wikipedia.org/wiki/Sof_Omar_Caves .

For birders, the reason for going there is that the dry valley above the cave entrance is one of only two reliable places to find the endemic and globally threatened Salvadori's Serin. We did find the bird but it did not oblige with a photograph.