weak layer of the mantle
Sitting immediately below the lithosphere, the asthenosphere corresponds to the depth range within the Earth where the melting temperature is most closely approached. Therefore, it is composed of hot, semi-molten deformable rock, within which convection currents occour.
The top is near the surface beneath oceanic ridges, around 120 - 180 kilometres deep under old ocean beds, and at least 250 kilometres deep, if present at all, beneath continents.
The asthenosphere also occupies roughly the same area of the mantle in which seismic waves move at low velocity.