The area that indicates a dramatic decrease in material density inside the Earth is the boundary between the mantle and the outer core.

Here's why:

* Mantle: The mantle is primarily composed of silicate minerals like olivine and pyroxene. These minerals are dense, but not as dense as the materials found in the core.

* Outer Core: The outer core is primarily composed of liquid iron and nickel. These metals are much denser than the silicate minerals in the mantle.

The sudden transition from dense silicate minerals to even denser liquid metals creates a significant decrease in material density. This change is detectable by studying seismic waves, which travel faster through denser materials.

This boundary is also known as the Gutenberg discontinuity.