S-waves (shear waves) require a solid medium to propagate and cannot travel through liquids. They are completely absorbed at the core–mantle boundary (Gutenberg discontinuity) at ~2,900 km depth, because the outer core is liquid (molten iron-nickel alloy). This creates a seismic 'shadow zone' for S-waves on the far side of the Earth from an earthquake epicentre. This observation provided the first evidence that Earth's outer core is molten.