Sof Omar Cave is the longest cave in Ethiopia at 15.1 kilometres (9.4 mi) long; 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 (6°55′N 40°45′E), 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. According to tradition Sof Omar was the name of a Muslim holy man who lived in the area and Ayiew the name of his daughter. Maco and Holuca are local names for ‘name’ and ‘cave’, respectively. 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’.
Approaching from Goro, at Sof Omar the scrubby bush steeply drops 90 m into a canyon. The Web river makes its way from the 4,300 metres (14,100 ft) high Bale Mountains through a 150 kilometres (93 mi) wide outcrop of Anatole limestone to the cave. In earlier times the river made a sharp left meander. At some point the limestone dissolved producing a series of phreatic passages, which became big enough to capture the whole flow of the Web river. Eventually the river abandoned the meander, creating a dry valley running from the cave sink at Ayiew Maco to the resurgence at Holuca. Sof Omar village is situated close to Ayiew Maco in the dry valley. Infill into the valley makes it rise to a high point of about 45 m above the Web, before it drops away to a pebble beach downstream of Holuca.
The other dominant feature is a large shakehole 100 metres (330 ft) wide and 60 metres (200 ft) deep and found on the basalt plateau directly above the cave.
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 approximately 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 1 m and 10 m. 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 continuation crosses the Web at Ford 1 and follows the figure-eight passage until the river is reached 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 15 m wide, 20 m high rectangular passage for 300 m.
The way onward 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 ends after Ford 5 under the 50 m high Great Dome. On the opposite bank a steep rise leads to Molossadie Passage. The deep, short Ford 6, Ford 7 and Ford 8 lead 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 the Molossadie passage and bypasses Fords 7 and 8 into the Chamber of Columns.
The 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 100 m downstream. The passage 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 200 m to the Big Rapids. This striking feature is formed from a jumble of huge boulders, well worn by the actions of the river. The river passage continues around a curve for about 250 m before flowing around a massive boulder into the sunlight at the Holuca Resurgence. The Web continues running through a canyon. About 250 m downstream the dry valley appears on the left bank.