The Mohorovicic (MOHO) discontinuity (first observed by Andrija Mohorovicic in 1909) is the boundary between the Earth's crust and the mantle. This boundary has been determined seismologically. Oceanic and continental crust are formed by entirely different geological processes: the former is typically 6-7 km thick, the latter about 35-40 km. Now for the first time, scientists have been able to drill into the lower section of Earth's crust, and will break through to the mantle (through the Moho) in coming years.

Earth composition (Courtesy: EJGE)The project, known as The Integrated Ocean Drilling Program (IODP) has drilled about 4,644 feet (1,416 meters) below the ocean seafloor (near Atlantis Massif, located at the intersection of the Mid-Atlantic Ridge and the Atlantis fracture zone). The seafloor was chosen because the crust is quite thin there, and therefore easier to breach. It took nearly 8 weeks to drill this hole, which is the third deepest ever made. The rock collection brought back to the surface is providing new information about the planet's composition.
The rocks brought back from that depth is already allowing scientists to form new theories and confirm old ones about how the Earth's crust formed, and how it is going to evolve in the future. This study has practical benefits as well: we will be better able to predict and analyze earthquakes once we know what the crust (and the mantle) are made of.
The next step is breaking into the mantle. The mantle material will be evident when and if it's brought up because it will have different texture and chemistry and will contain different proportions of minerals compared with rock in the crust.