Context: Around the 1920s, German architect Herman Sörgel came up with an idea to drain water from the Mediterranean sea by building a dam across the strait of Gibraltar, among other places. The goal was to reduce the sea level enough to claim more land from the sea and bring Europe closer to Africa so that Europe could colonize it and hedge against the growing power of the US post WW1. However, with the fall of colonialism and invention of nuclear power after WW2, the idea lost support.
It was a combination of not having enough science to realize the problems or costs, and a bit of racism. I haven't seen Tom Scott's video, but AlternateHistoryHub did a short video that includes this what if, and then wrote a book about it because it became far more interesting that initially thought.