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) and is relatively dense.

* Outer core: The outer core is composed of liquid iron and nickel. While iron and nickel are denser than silicates, the liquid state allows for greater packing efficiency, making the outer core less dense than the solid mantle.

This density difference is a major factor in the Earth's magnetic field. The liquid outer core's movement generates electric currents, which create the magnetic field that surrounds our planet.