Lac Abbé is a salt plain on the border of Djibouti and Ethiopia with hundreds of limestone chimneys standing on it, some of them fifty metres tall and a few still steaming. They were all built underwater. Hot springs rising through the lakebed laid calcite around their vents, and a tower could only grow while it was submerged. The lake made them and then went away. Lac Abbé is where the Awash river ends, coming down from the Ethiopian highlands and never reaching the sea, and drought and irrigation upstream have pulled the surface far back. What is left is cracked salt, with nothing on it but the towers and nothing beside them to give their size until you stand at the foot of one.
The ground under all this is coming apart. Three plates — Arabian, Nubian and Somali — are separating here fast enough for geologists to watch, and this is one of the few places on land where an ocean is being made. In a few million years the lowland is expected to flood and cut the Horn of Africa off from the rest of the continent. For now the vents still exhale steam through the crust, and some of them are hot enough to burn you. The drive in crosses open plain shared with camels, wild donkeys and Afar herders moving goats. You sleep in a camp of toukouls, dome huts of bent branches and woven mats set out in the open with no electric light for kilometres. Dawn is the hour: the towers go black against the sky, the steam shows, and flamingos work the saline shallows before the heat arrives. Guides will tell you the first Planet of the Apes was filmed here. It was shot in Utah and Arizona; the story stuck because the place looks the part, and the guides mostly got it from visitors. The chimneys only ever grew below the waterline, so the tallest of them stand where the lake used to.