Some marine cooling systems use chilled seawater as a direct cooling agent as an alternative to thermal transfer fluid systems. In a water jacket cooling system, seawater is pumped directly into a chamber, known as the jacket, that surrounds engine cylinders. Heat transfers from cylinder walls to the water as it cycles through the jacket chamber. In some large-scale marine cooling systems, warm water returning from the engine can either be cooled and recirculated or returned to the source by pumping overboard.