It's amazing to think that we may be spending $1 billion to drill through the Moho, when you consider that a century or so ago, we didn't even know that boundary existed. In 1909, Andrija Mohorovičić, a Croatian researcher, discovered that about 20 miles (50 kilometers) inside the Earth, the waves triggered by earthquakes traveled faster than they do closer to the surface. While scientists had at least a vague notion already that the Earth had layers, Mohorovičić's work suggested that there was a clear boundary between the crust and a layer beneath it that had different composition and physical properties. In his honor, we now call that boundary the Moho [source: Osman].
Since then, scientists have managed to learn a bit more about the mantle, the layer that lies beneath the Moho, which amounts to 83 percent of the Earth's volume and 67 percent of its mass Encyclopaedia Britannica. The easiest way to understand this is to think of the Earth as a chocolate éclair. The thick outer layer of glazed chocolate and baked dough is solid but elastic. That's the crust. Underneath that, though, there's a lot of viscous, gooey stuff. Of course, that's a limited analogy, because the Earth isn't cream-filled. Instead, the mantle is made of molten, fluid rock called magma. Some of that magma is ejected by volcanoes, so we know that in the upper part of the mantle — that is, the top 620 or so miles (1,000 kilometers) — it seems mostly to be composed of oxides of silicon, magnesium and iron, with smaller amounts of aluminum oxide, calcium oxide and alkalies thrown into the mix [source: Encyclopedia Britannica].
That said, our knowledge of the mantle is fairly limited. Scientists can't go down and look at it, and they've never had a pure sample taken directly from the deep to analyze. That's what the 2012 MoHole to the Mantle project hopes to accomplish.