The boundary between the mantle and core is called the Gutenberg discontinuity. It is located at a depth of about 2,900 kilometers (1,800 miles) below the Earth's surface.

Here's what makes this boundary significant:

* Compositional Change: The mantle is primarily composed of silicate minerals, while the core is primarily composed of iron and nickel. This abrupt change in composition creates a distinct seismic signature.

* Density Change: The core is much denser than the mantle. This density change is also reflected in seismic waves, which slow down as they pass through the Gutenberg discontinuity.

* Seismic Wave Behavior: P-waves (pressure waves) can travel through both solid and liquid, while S-waves (shear waves) can only travel through solids. S-waves are stopped at the Gutenberg discontinuity, confirming that the outer core is liquid.

In summary, the Gutenberg discontinuity is a physical boundary marked by changes in chemical composition, density, and the behavior of seismic waves, which helps us distinguish between the Earth's mantle and core.