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

Here's why:

* Mantle: The mantle is primarily composed of solid rock, albeit a very hot and viscous kind. This rock is made up of silicate minerals, which are relatively dense.

* Outer Core: The outer core is made of liquid iron and nickel. Iron and nickel are significantly denser than the silicate minerals found in the mantle.

* Density Change: The transition from the solid, silicate-rich mantle to the liquid, iron-nickel outer core results in a dramatic decrease in density. This change in density is also reflected in the seismic waves traveling through the Earth, as they slow down dramatically at this boundary.

Let me know if you'd like to know more about the structure of the Earth!