If the Chikyu's drill rig breaks through a transitional boundary called the Moho, it would reach the Earth's mantle, the mysterious 1,740-mile (2,900-kilometer) thick layer between the crust and the planet's hot, molten core [sources: USGS, ScienceDaily].
http://science.howstuffworks.com/environmental/earth/geophysics/dig-hole-to-earths-mantle.htm
This may be about to change with a $1 billion mission to drill 6 km (3.7 miles) beneath the seafloor to reach the Earth's mantle -- a 3000 km-thick layer of slowly deforming rock between the crust and the core which makes up the majority of our planet -- and bring back the first ever fresh samples.
http://www.cnn.com/2012/10/01/tech/mantle-earth-drill-mission/index.html
Scientists planning mission to drill down to Earth's mantle and bring back first fresh samples. Mantle could hold clues to origin and evolution of the planet. Geologist calls it the most challenging endeavor in the history of Earth science. If project gets the go-ahead, the team hope to reach the mantle by early 2020s. 1  Humans have reached the moon and are planning to return samples from Mars, but when it comes to exploring the land deep beneath our feet, we have only scratched the surface of our planet.
http://www.cnn.com/2012/10/01/tech/mantle-earth-drill-mission/index.html
Geologists have had to contend with bad luck, budget cuts and the race to the moon in their efforts to drill deep into our planet. The drill bit that the Atlantis Bank expedition broke near the start of operations. Three of the four cones used to dig the hole have snapped off.
http://www.smithsonianmag.com/science-nature/decades-long-quest-drill-earths-mantle-may-soon-hit-pay-dirt-180957908/
[–]iorgfeflkdBiophysics 263 points264 points265 points 3 years ago (129 children) The deepest we have dug is the Kola Superdeep Borehole in Russia which went about 12 km deep. There have also been some oil wells that have gone similarly deep.The deepest known cave is the Voronya cave which is about 2 km deep. permalink.
https://www.reddit.com/r/askscience/comments/1ko9a7/how_deep_into_the_earth_have_humans_explored/
While the Kola Super deep Bore hole has not only been the deepest, but also the longest borehole in the world for years, in May 2008, a new record of length has been established by extended reach drilling (ERD) well drilled by Transocean for Maersk oil in the Al Shaheen oil field in Qatar.
http://www.innovateus.net/earth-matters/what-deepest-hole-ever-drilled
But still, somewhere in the back of your mind, you've kept wondering about what would happen if someone dug a really, really deep hole. Well, you may not have to wonder any more, if an international team of scientists who call themselves the 2012 MoHole To the Mantle project succeed in their quest.
http://science.howstuffworks.com/environmental/earth/geophysics/dig-hole-to-earths-mantle.htm
In this article, we'll explain the difficulty of digging such a deep hole, and what we might gain from it. 1  1. 2  2. 3  3. 4  4. 5  5.  NEXT.
http://science.howstuffworks.com/environmental/earth/geophysics/dig-hole-to-earths-mantle.htm
Meanwhile, the earthbound geologists who dreamt of getting a glimpse of Earth's inner workings were left empty-handed with the remnants of various programs thanks to budget cuts. Since the 1960s, researchers have attempted to drill into Earth's mantle but have not yet met with success. Some efforts failed due to technical problems; others have fallen prey to various sorts of bad luck—including, as discovered after the fact, picking inopportune spots to drill.
http://www.smithsonianmag.com/science-nature/decades-long-quest-drill-earths-mantle-may-soon-hit-pay-dirt-180957908/
The deepest hole ever drilled by humans (Russians) is the “Kola Super deep Borehole”, on Kola Peninsula in the North West corner of Russia, located near Finland”. Russian scientists had to invent new ways of drilling, due to the depth of that area. A number of bore holes were drilled by branching from a central hole.
http://www.innovateus.net/earth-matters/what-deepest-hole-ever-drilled
how deep have we drilled
We have drilled 1,740 miles deep. We have drilled up to 1,740 miles.