Geologists have embarked upon an ambitious project to drill through the earth's crust and extract a sample of the mantle.
Fifty years after the first, unsuccessful attempt, scientists from the UK's National Oceanography Centre and France's Montpellier University plan to start drilling next month.
Previous expeditions have made the job much easier by drilling down more than 1.5 km below the sea floor, where the crust is only eight to ten kilometers thick, compared to 30 to 60km on the continents.
It means there's only another 400 meters or so to go - although the attempt could still take as long as 15 years to complete, the scientists say.
The team has chosen three possible sites: off the coasts of Hawaii, Baja California and Costa Rica. The Pacific was chosen becaise the crust there formed faster than in the other oceans and therefore has a more uniform structure, making it simpler to drill.
The aim of the exercise is to extract samples of the earth's mantle, which makes up 68 percent of the earth's mass.
"If successful, this would be the first in situ sampling of the largest part of our planet," says Damon Teagle, a geochemistry professor at the University of Southampton, and co-author of a report appearing in Nature.
Until now, scientists have only been able to study the earth's mantle by examining samples that have been thrust to the surface by geological processes or spewed out by volcanoes - and these have been affected by this activity and may have changed in composition.
The drilling will be carried out by a huge Japanese deep-sea drilling ship, Chikyu. However, Teagle says the team will need to develop new tools that can withstand the intense pressure and heat of over 300 degrees Celsius.