Unlike traditional swimming pools, which contain chlorine, natural pools rely on plants and rocks for filtration instead. An underwater retaining wall separates the swimming area (where humans and sometimes fish swim) from the regenerative zone, which closely resembles wetlands filled with aquatic plants like water lilies or water lotus. Pumps, and sometimes waterfalls, keep the water moving, and the rocks and skimmers filter sediment and large debris.The alternative pools gained popularity in the UK before the trend was imported here. Costs range from between $50,000 to $200,000 to install and have the added bonus of self-winterization. One drawback might be the intrusion of some unwanted animal guests; however, theTimes says: "Natural pools are considered lower maintenance, too, because owners dont need to add chlorine once a week to the water. Other issues may arise, such as too much algae, but generally, it comes down to the plants, which can be thinned out."