What lies deep beneath the ocean?

There is one immense place in our planet we have never reached-Earth's mantle. Much of our knowledge about Earth's interior comes from interpreting measurements of seismic waves as they travel around and through the globe. These studies reveal that, deep below the surface, below Earth's crust, seismic waves abruptly change speed and direction. We call the region of this transition the Moho-short for the Mohorovicic discontinuity-and it marks the beginning of the mantle. Humans have flown to the moon and retrieved samples from an asteroid, but we have still not even attempted ultra-deep drilling into the mantle.
The mantle?last vast frontier of planet Earth

The mantle?composed of solid and dense rocks- is the largest layer of Earth. 2,900 kilometers (1,800 miles) thick, it makes up about 83% in volume and 67% in weight of our planet. Within Earth's outer crust rocks remain rigid. Deeper, the temperatures and pressures increase until rocks soften and deform. On this viscous material, the crustal tectonic plates slide and collide over millions of years, forming new seafloor, building continents, and causing great earthquakes as crust sinks back into the mantle. These slow movements also exchange important substances-including water, carbon, and oxygen- among the deep Earth, ocean, atmosphere,and even biosphere.
Illustration demonstrating some of the most important underlying crustal processes that formed and continue to shape our planet's surface and crustal geology. Great strides in our understanding of these crustal processes will be achieved by studying the nature and composition of directly sampled deep crust and mantle material up-to and beyond the Moho (figure courtesy of Smith-Duque C.E).