Solar ponds are large, shallow, human-made bodies of water lined with dark material. The pond has three main water layers. From top to bottom, these layers are called the surface zone, the gradient zone, and the storage zone. The salt content of the pond increases from top to bottom. Water in the storage zone is extremely salty. As solar radiation is absorbed the water in the gradient zone cannot rise, because the surface-zone water above it contains less salt and therefore is less dense. Similarly, cooler water cannot sink, because the water below it has a higher salt content and is denser. Hot water in the storage zone is piped to a boiler where it is heated further to produce steam, which drives a turbine.