The "D Layer" refers to a specific layer within the Earth's mantle, located just above the outer core. It is characterized by its unique mineral composition and physical properties, which differ from the layers above and below it. This layer plays a crucial role in the dynamics of the Earth's interior, influencing tectonic activity and heat transfer. Scientists study the D Layer to better understand the processes that drive plate tectonics and the formation of volcanoes. Its depth ranges from about 2,500 to 2,900 kilometers beneath the Earth's surface, making it a significant area of interest in geology and seismology.