Since old times people - especially geologists - speculated about the interior of Earth. The Italian poet Dante Alighieri (1265-1321) imagined an allegoric center of the Earth: a frozen wasteland, not reached by the divine light, where Lucifer is entrapped in eternal ice.
The French Sci-Fi author Jules Gabriel Verne (1828 - 1905) based "A Journey to the Center of the Earth" (1864) already on early science. In his novel Verne uses the hollow conduit of an Icelandic volcano to venture inside earth, an idea supported by the geologic models of volcanoes proposed at the time - a single or a series of magma chamber(s) with conduits connecting them to the surface. Geologists assumed that during an eruption the magma reservoir becomes empty and large voids and caverns were left behind.
Fig.1. This geological section, published in the book by German professor of geophysics August Sieberg "Einfhrung in die Erdbeben- und Vulkankunde Sditaliens" (1914), shows the anatomy of a stratovolcano, with a main conduit, various lateral dikes and a large sill connected to the magma reservoir. In contrast to the sketch, the conduits for magma are in reality only a few meters wide - too small for travel the Center of the Earth (image in public domain).
Since the 19th century many of Verne's visions became true - humans visited the moon, submarines can travel under the sea and travels around the world are no longer a privilege for rich gentlemen - but what about the universe within earth?
The deepest natural cave known today is the Cave of Kruber (also Cave of Voronja), located in the Arabika Massif of the Gagrinsky Range of the Western Caucasus. This cave is explored to a depth of 2.191m, but possibly continues.
The deepest mines in the world are the TauTona and Savuka gold mines in the Witwatersrand region of South Africa, which are currently working at depths exceeding 3.900m.
In May 1970, to celebrate the birthday of Lenin, the former Soviet Union initiated the secret project "SG-3" on the Kola-Peninsula. The drilling project planned to study the Mohorovičić discontinuity, situated at a depth of 15 kilometers. Thee project continued until 1989, when technical and financial problems stopped the drill at 12.261 meters.
The United States initiated a similar ambitious project, but decided to drill the thinner oceanic crust (5-10 kilometers thick). Project Mohole started in 1961 and was abandoned in 1966, after recovering 170 meters long cores from the ocean floor in a depth of 3.500 meters. Modern commercial boreholes reach depths of 2.000-3.000 meters.
However even the Kola borehole covers just 0,2% of earth's radius.
During a meeting of the International Association of Geologists in 2013 the chairman of the association, Dr. Abner Perry, lamented this fact "Well gentlemen, at one point at least I agree with Professor Christophe, the materials of the geologists are not charts, chalk and chatter, but the earth itself. We should never know the truth, until we are able to make that journey, and see for ourselves."
Perry proposes the development and construction of a drilling machine to go to the earth's core in this decade. Sourcing would be done by an anonymous US financier, however the association is now searching for brave "terranauts" willing to take the risk of this fantastic voyage. Interested candidates can read about the project following this link.
CARLSON, D.H.; PLUMMER, C.C. & HAMMERSLEY, L. (2009): Physical Geology - Earth Revealed. 9th ed., McGraw-Hill Publisher: 645
SCHICK, R. (2002): The Little Book of Earthquakes and Volcanoes. Springer/Copernicus Books, New York: 164