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 2nd longest in the World. It is situated in the Bale province in southeastern Ethiopia through which the Weyib 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 local traditional religions and to Islam. The caves are known for their many pillars, particularly in the "Chamber of Columns".

Topography

When you reach Sof Omar from Goro the scrubby bush steeply drops 90 m into a canyon. The river Web makes its way from the 4,300 metres (14,100 ft) high Bale Mountains and 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 cave dissolved a series of phreatic passages, which became big enough to capture the whole flow of the river Web. 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 the large shakehole 100 metres (330 ft) wide and 60 metres (200 ft) deep. This is found on the basalt plateau directly above the cave.

Cave description

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. The river passage continues around a curve for about 250m before flowing around a massive boulder into the sunlight at the Holuca Resurgence. The Web continues running through a canyon. About 250m downstream the dry valley appears on the left bank.

The left side of the canyon from the resurgence to the dry valley is riddled with passages breaking out into the canyon walls, forming many of the unusable entrances to the cave. Most of these are small passages exposed as the river cut its way down. The exception to this is the 25m wide Great Hall that follows the north – south joint pattern breaking into the canyon at the resurgence. Hidden at the end of the Great Hall behind boulders is the hard-to-find Chameleon Passage. This runs along the east–west joint pattern. Some small passages lead from Chameleon to Blank Passage and back to the Big Rapids. Several small passages run from Blank Passage to Boulder Chokes.

The area surrounded by the river, Great Hall, Chameleon and Blank Passage contains a criss-cross labyrinth of passages. Two of the most significant, Bill's Passage and Tautology Passage, run between the river and the Great Hall. These are high rift passages. By climbing them it is possible to access the 35m long Balcony that overlooks the Big Rapids 7m below.

Running parallel to the Great Hall, and interconnected to it via several passages is Flintstone Passage. This splits into lots of different passages that break into the left bank of the canyon downstream of Holuca. Away from the river Flintstone runs into another of the caves' unique passages – Mudwall Passage. This passage has been filled with mud virtually to the 3m high roof. Following the infilling process a vadose trench has been cut the length of the passage revealing that the mud is formed of thin laminations.

After 200 m the Mudwall Passage enters into the small Mudwall Chamber. This has been formed by water coming from Link Passage on the west cutting across the line of the Mudwall Passage disappearing into Rimstone Passage to the east. It is this flow of water that formed the chamber. Climbing out of Mudwall Chamber leads to Batshit Passage whose floor is knee deep in guano. This leads to the large chamber, the Astrodome, which houses a large colony of bats.

Link Passage is a narrow rift passage that links back to the Chamber of Columns. A series of small passages at two levels leave Link Passage and heads towards the river passage. One of these leads into the wide Blind Passage, which also emerges into the Chamber of Columns. The upper passage is a tight crawl into the pretentiously named Wembley Stadium. This passage is formed in a fault. The survey of the British expedition in 1972 showed that the Wembley Fault extended from a right bank passage on the river, to Wembley Stadium, Mudwall and Rimstone Passages.

The Shakehole on the plateau centred on the Wembley Fault line. Obviously the fault played a significant role in the major collapse that created the Shakehole. The underground debris from the collapse forms the Big Rapids and is responsible for the boulder chokes seen near Blank Passage.

credited to wikipedia and flickr: flo142