Roughly 6,000 years ago, the middle of what is now the southern Sahara held a lake larger than all five North American Great Lakes combined, spread across about 361,000 square kilometres of open water. Today, only Lake Chad remains as the surviving surface-water remnant of that much larger system, shared by Chad, Niger, Nigeria and Cameroon, while the ancient lake basin stretches far beyond it. At its highstand, palaeolake Mega-Chad was about 48 per cent larger than the combined surface area of the five Great Lakes, which together cover 244,106 square kilometres. The story of how a lake that size formed, and how it disappeared, comes from old shorelines etched into the desert landscape and sedimentary deposits that scientists have dated using optically stimulated luminescence and other geological methods, revealing a climate shift far larger and faster than most people would expect from a region now defined largely by sand. Its disappearance also left a landscape that records how strongly rainfall can reshape entire regions over relatively short geological timescales.
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