At 15.1 kilometres (9.4 mi) long, Sof Omar Cave is the longest cave in Ethiopia; sources claim it is the longest system of caves in Africa and ranks as the 306th longest in the World. It is situated to the east of Robe, in the Bale Zone of the Oromia Region in southeastern Ethiopia through which the Weyib River (Gestro River) flows. It sinks at the Ayiew Maco entrance and reappears at the Holuca resurgence 1 kilometre (0.62 mi) away. Long a religious centre, it is sacred both to Islam and the local Oromo traditional religion. The caves are known for their many pillars, particularly in the "Chamber of Columns".
The cave is formed along a network of joints: one set runs approximately north to south and the other east to west. This zig-zag of passages runs in an approximate southeasterly direction. Sof Omar has 42 entrances, but generally only four are useful for gaining entrance:
- Two upstream Village Entrances (one to the east and one to the west of the village)
- The Tourist Entrance downstream from the Holuca Resurgence - at a point where the abandoned meander forming the dry valley rejoins the Web river
- A right bank entrance downstream of Holuca accessing the Deep South part of the Clapham's Climb Series
Entering the cave via either of the Village Entrances the visitor passes a shrine used by the locals. The Ayiew Maco Series is a set of interconnecting passages of varying in width between 1m and 10m. Several can be passed through to the pebble beach on the left bank of the river. A less complex series of passages exists on the right bank. These probably connected to those on the left bank until severed by the vadose action of the Web cutting the river passage deeper.
The passage at the pebble beach is about 40 m wide - the widest passage in the cave. At the downstream end of the beach the river disappears between two columns. The way to continue is to cross the Web at Ford 1 and follow the Figure-of-Eight passage until reaching the river again at Ford 2. At this point it is possible to see down Safari Straight, the most spectacular view in the cave. The river meanders down this 15m wide, 20m high rectangular passage for 300m.
The way on involves crossing and re-crossing the river from cobble beach to cobble beach at Fords 2, 3, 4 and 5. Small passages exist on both sides. The beach after Ford 5 ends under the 50m high Great Dome. On the opposite bank a steep rise leads to Molossadie Passage. The deep, short Ford 6, Ford 7 and Ford 8 leads to a cobble beach and the entrance into the Chamber of Columns. Exiting Ford 6 on the left bank it is possible to enter the Railway Tunnel passage which offers an alternative route into Molossadie and a bypass to Fords 7 and 8 into the Chamber of Columns.
Chamber of Columns is a unique feature in the world of caves. It looks like a wide passage leaving the Web before sweeping back to the river about a 100 m downstream. The passage is circumnavigates a cluster of thick columns densely packed in the centre of chamber. The Railway Tunnel and Molossadie Passage enter the chamber from the north. The river passage continues from the Chamber of Columns meandering for about 200m to the Big Rapids. This striking feature is formed from a jumble of huge boulders, well worn by the actions of the river.