A thermocline is a layer in a large body of water, such as the ocean or a lake, where the temperature decreases rapidly with depth. It typically occurs around 1,000 meters deep and separates the warmer, sunlit upper layer from the cooler, darker deep water below. This layer is important for maintaining balance and regulating the temperature of the water. Thermoclines are found in all bodies of water and have a significant impact on the Earth's climate.