What it was
Picture an apple with a hot metal heart. Earth's crust is thin compared with the globe, like the skin of that apple or even thinner. Under the ocean and the ground you walk on, most of the planet lies out of reach.
Earth grew from rubble crashing together around the young Sun. The blows melted rock. Heavy metal sank toward the middle as more rubble arrived, so the world sorted itself in stages. Steam wrapped it in a blanket that slowed its cooling.
When and where
Our planet began forming in the inner Solar System about 4.54 billion years ago. The show's Clock puts that start on January 1. As Earth cooled, patches of crust held firm and rain could collect on top. A sea had somewhere to sit.
How big
Earth spans about 7,918 miles (12,742 kilometers). Beneath its thin crust is the mantle, a deep layer of mostly solid rock. Solid does not mean still: it can flow very slowly. The metal core sits below that rock, with a liquid outside and a solid center.
How we know
Earthquakes send waves through the hidden layers. Their speed and paths change as they meet different stuff, letting scientists map what no drill can reach. At the surface, magnetic grains in old lava keep compass marks from the past. Changing atoms in rocks keep clocks.
Still a mystery
Scientists still argue about when Earth's plates began to move. Earth keeps wrecking the clues: rock melts or wears away, and old seafloor sinks back down. Some of the earliest traces now fit inside tiny crystals, loose grains caught in younger stone.