The asthenosphere is a layer of the Earth's mantle that lies beneath the lithosphere. Its depth range is not fixed, but it is generally considered to be between 100 km (62 mi) and 700 km (435 mi) below the surface.

Here's a breakdown:

* Upper limit: The asthenosphere begins at the base of the lithosphere, which can vary in depth depending on tectonic activity and location.

* Lower limit: The asthenosphere transitions into the deeper, more rigid mesosphere. The precise depth of this boundary is also variable.

Important notes:

* The asthenosphere is not a solid layer, but a zone of partial melting. This means it has a weak, plastic-like consistency that allows for slow, ductile flow.

* The exact depth and characteristics of the asthenosphere can vary depending on factors like tectonic activity, temperature, and pressure.

* It's important to remember that the Earth's interior is not uniformly layered. There are transitions and complexities within each layer.

So, while the range of 100 km to 700 km is a general guideline, the asthenosphere's exact depth can vary considerably.