The process of taking the salt out of seawater, or desalination, is as old as human civilization. Socrates taught students how to distil seawater to obtain fresh water. Sailors have been applying similar methods for over 2,000 years. Solar ponds that were developed to harvest salt have let nature do the distilling for thousands of years already. The sun's heat was used to desalt seawater, which served as the drinking water for troops during the siege of Alexandria in the days of Julius Caesar.