The Earth is made up of four basic layers: the crust, the mantle, the outer core, and the inner core. Even at its thickest point, the crust is less than 50 miles thick. The core lies at the center of the planet, roughly 1,800 miles through the mantle. But even at those immense depths, you’ve still only reached the outer core, which is itself almost 1,400 miles deep, according to National Geographic. And between the mantle and the outer core is a boundary that’s known as the Gutenberg discontinuity.
According to EOS, the outer core is a liquid “rolling mass of molten metal” that surrounds the solid inner core. The inner core is found 3,200 miles below the surface of the Earth and is almost 1,550 miles wide, roughly the size of the Moon. For a while, it was believed that the Earth’s core was a chunk of solid iron. But seismic measurements from as early as the 1950s suggested that it might contain more than just iron. The core is actually about 10% less dense than iron, and most likely also contains, in addition to other elements, nickel, silicon, and magnesium. Sulfur is also found in the core, and it’s estimated that roughly 90% of the Earth’s sulfur is located there.
And funnily enough, the current makeup of the core is actually younger than Earth itself, having formed between roughly half a billion and 1.5 billion years ago.