Layer of saline clay and clear salt here is some 50 - 100 m deep. During the rainy periods salt flats turn into endless shallow lake - in fact five rivers flow into these plains.
Kubu Island, Botswana Kubu Island / Stefan Huchler, Wikimedia Commons, CC BY-SA 3.0
Kubu Island itself is not an island - it is an outcrop of 2 billion years old granite which is surrounded from three sides by the Sua (Sowa) pan of Makgadikgadi salt flats. This hill is approximately 1 km long and rises up to 20 m above the salt flats. Granite is covered with fossilized guano from the times when the island was surrounded by lake and birds used to be on the island.