The Great Sahara Sea

According to this article on IO9, in the 19th century there were plans to dig a canal from the Mediterranean to flood the low lying areas of the Sahara desert and create a giant inland sea.

The London Times said that the plan "dazzles the imagination, yet it has a sufficiently substantial basis to satisfy several shrewd traders in African commerce and some distinguished engineers." The Daily News called the project "the most remarkable that has ever been devised."