Jules Verne was an early pioneer of science fiction, and his final published novel, Invasion of the Sea, has perhaps the most far-fetched gimmick in his entire body of work: a European engineer named de Schaller travels to Tunisia with plans to flood the Sahara, turning the world's largest hot desert into a vast inland sea. The most surprising part is that Verne didn't dream up this crazy scheme: the idea of creating a "Sahara Sea" has been floated by scientists for 140 years, and almost happened as recently as the 1980s.

The Sahara has been pretty depressed lately.
The only thing that makes these schemes possible is that vast tracts of the Sahara Desert are actually below sea level. The El Djouf basin of Mauritania, the chotts (dry salt lakes) of Tunisia, the Qattara Depression in northwest Egypt—all are some of Earth's lowest places on dry land. The bottom of the Qattara Depression is 436 feet below sea level, making it Africa's second lowest point.

From the people who brought you the Suez Canal… 
A Saharan sea was first proposed in 1877 by the British engineer Donald McKenzie, who realized that a 400-mile channel from Morocco's Cape Juby southeast into Mauritania would create an inland sea the size of Ireland. Ferdinand de Lesseps, the French diplomat who helped build the Suez Canal and (before America bought him out) the Panama Canal, saw that central Tunisia was a much easier place to flood. His lake would have been smaller, about the size of Massachusetts.