Smithsonian.com has an interesting article about the deepest hole ever dug. The target is the Earth's mantle. The mantle makes up 40 percent or more of the whole planet. The Earth has a radius of 4000 miles. The mantle accounts for about 1800 miles of that, which compares to 3 to 25 miles thick for the crust.
“If we have a better knowledge of what the mantle is and how the mantle behaves, we have better knowledge of volcanoes and earthquakes, and better knowledge of how the planet as a whole works,” said Benjamin Andrews, a research geologist and a curator for the National Rock and Ore Collection at the Smithsonian's National Museum of Natural History.
Scientists took their first crack at the mantle in 1958 with Project Mohole. American engineers drilled through the Pacific Ocean floor off Guadalupe, Mexico. But Congress discontinued funding in 1966 before the drillers ever reached the mantle.