National Geographic: The oceans have long taunted those who thirst.
Records dating to A.D. 200 show that sailors boiled seawater and used sponges to absorb fresh water from the steam. Today, desalination is more sophisticated: multistage flash distillation, reverse osmosis, electrodialysis, and more.
But one thing hasn't changed since the time of the ancient mariners: It takes a lot of energy to squeeze drinkable water from salt water. So even though more than 70 percent of the Earth's surface is covered with water,...