Salt-gradient solar ponds are used to collect and store solar energy. The upper zone acts as an insulator, allowing sunlight to heat the lower zone up to 95°C. A 3m deep pond of 150m2 can provide all winter heat for a house in Columbus, Ohio. These ponds are found in nature and have been studied since 1958. Recent research shows they are easy to operate and can provide low-cost heat at $0.010 per kW-hr thermal. Experiments are ongoing to confirm these cost estimates.