The asthenosphere is a layer of the Earth's mantle located beneath the lithosphere, extending from about 100 to 410 kilometers (62 to 255 miles) below the surface. It is characterized by its semi-fluid, plastic-like properties, allowing it to flow slowly over geological time. This layer plays a crucial role in tectonic plate movement, as the lithospheric plates float and move atop the more ductile asthenosphere. Its behavior is essential for understanding geological processes such as earthquakes and volcanic activity.