A manhole, as you more than likely know, is a covered aperture upon a street or footpath to allow access to the utilities beneath. At least I suppose this is why such things exist, although I must admit, I have never thought about it very much. And a mole-hole, of course, is where a mole resides. But a Mohole, quite literally, is very much a deeper concept: to put it simply, it is a hole that is deep enough to reach the Moho.
Let us start at the very kernel of the issue. At the centre of the Earth lies the liquid core, slightly over 4,000 miles in diameter. Its composition is not known for sure, but it is thought to be a mixture of iron and nickel. Neither has the temperature at the Earth's core been measured directly, but on the basis of the rate at which temperature is known to increase with depth in very deep mines, and the rate at which rocks can conduct heat, geologists guess that temperatures in the liquid core must be in excess of 5,000C.
The core is separated from the next layer of the earthly onion, the mantle, by what is called the Gutenberg Discontinuity. The mantle is some 1,800 miles thick, and is composed of a type of rock rich in magnesium and iron, known - because of its dark green colour - as olivine. It stretches to roughly 20 miles from the Earth's surface - and brings us to the Moho. The outer shell of our planet is the crust. Its thickness varies from as little as three miles under the deep oceans to over 40 miles beneath very high mountains. The boundary between the crust and the mantle is known as the Mohorovicic Discontinuity, named after Andrija Mohorovicic, a Croatian scientist who lived in what we used to know as Yugoslavia, and who discovered it in 1909; not surprisingly the name was shortened, and the boundary has come to be known as the Moho.
The Mohole Project, as it was called, was one of the Great American Dreams of the early 1960s. It reflected the fact that although we know a great deal about what goes on above our heads, we know comparatively little about the composition of the Earth miles beneath our feet. The aim was to bore an exploratory hole 20 miles deep through the Earth's crust, deep enough to pierce the Moho, and thus allow scientists to examine the Earth's mantle directly for the first time. But this particular American dream was shattered in August 1966 when the US Congress blocked the Mohole Project for financial reasons.