Here's why:

* Earth's Core: The Earth's core is primarily composed of iron and nickel, with iron making up about 88% of the core's mass.

* Mantle: While the mantle is mostly composed of silicate minerals, it also contains significant amounts of iron, along with magnesium, silicon, oxygen, and calcium.

* Crust: The Earth's crust is the outermost layer and is much thinner than the mantle. It contains various elements, including iron, oxygen, silicon, aluminum, calcium, sodium, potassium, and magnesium.

While other elements are also present in various layers, iron is a key component of all of them, making it the element found throughout the Earth's structure.