Billions of years ago, a Mars-sized body smashed into the young Earth, causing a chain reaction that created the Moon. Earth's atmosphere became full of rock vapor and the sky rained down magma. Yes, it was literally Hell on Earth.
The collision between Earth and this Mars-like body was one of the most dramatic events in the history of our solar system, and the aftermath was even more incredible. As the destroyed planetoid became a disc of orbiting magma, it shared a common atmosphere with Earth unlike any we can imagine. This was an atmosphere of vaporized rock, which encircled both the magma-covered Earth and the newly forming Moon.
Because they shared a common atmosphere, the Earth and the Moon should share some of the same basic materials, but this does not appear to be the case. Instead, Moon rocks have way more iron and far less magnesium than their Earth counterparts, which does not fit the idea of a completely mixed common atmosphere.
Now researchers have an answer, and it's completely awesome. Magma rain would resolve the mystery, as rising rock vapor would see its magnesium oxide start to condense into droplets and fall back onto the planet's surface. The iron oxide inside the rock vapor wouldn't have condensed as easily, meaning far more of it got mixed into the disc that became the Moon.
This means that, for millions of years, unimaginably hot molten rock fell from the sky onto the infant Earth, and this was about as regular an occurrence as a rainstorm today. So, just to recap, the ancient Earth looked pretty much exactly like the cover of that heavy metal album you bought in the 80s, just with slightly fewer warrior queens in metal bikinis.