Ancient methods of desalination involved crude filters and capturing steam from boiling water, a practice which today has been improved on an industrial scale. But, again, the energy costs are enormous--it's one thing for ancient sailors to boil water on a ship at sea to get them through the day, it's another thing entirely to provide for the daily water needs of hundreds of thousands of city-dwelling people. Some desalination plants still start by boiling saltwater in a large chamber. Once that steam has lifted away from the salt, it is cooled at the top of the chamber and condenses, draining out into tanks for further filtration. Energy is required to both boil and cool the same water, making the whole process pretty inefficient. An alternative and more popular method for large-scale operations is reverse osmosis. In reverse osmosis, water is sent through filter after filter after filter at high pressure, hoping to remove more and more salt each time. Getting water through these filters is the most energy-intensive part of the process, and the thinner the filter, the less pressure you need.