The boundary between the mantle and the core is called the Moho (Mohorovitch discontinuity). Scientists first tried to drill the sea floor to reach this area during the Mohol project in the 1950s and 1960s, but were unsuccessful.
The pressure in the core of the earth is 350 million times the atmospheric pressure on the surface of the earth
A hole drilled in an attempt to drill a well into the ground will collapse unless we continuously pump a fluid known as drilling mud into the hole. In deep sea drilling and oil wells, this fluid is a mixture of mud that contains heavy minerals such as barium. The weight of the fluid balances the pressure inside the cavity with the pressure of the surrounding rocks and prevents the cavity from collapsing.
Drilling mud also plays two other roles: it cleans the mud, prevents the machinery from getting dirty with sand, and also helps to lower the temperature. Of course, it will be almost impossible to keep the drill cool in the innermost layers of the earth. For example, the temperature of the mantle is 1410 degrees Celsius. At this temperature, stainless steel melts, so the drill must be made of a special, expensive alloy such as titanium.
After passing through the mantle, the drill reaches the earth’s core at a depth of about 2896 km. The outer core is mainly made of molten nickel and iron and has a temperature of 4000 to 5000 degrees Celsius. Drilling this very hot alloy will be associated with many problems. Drilling a fiery outer core is like drilling in a liquid and will likely melt the drill bit unless cold water is pumped down.
After traveling 5000 km, the drill reaches the inner core. There, the pressure is so intense that despite the very high temperature, iron and nickel remain in a solid state. In this area, there are truly indescribable pressures of about 350 gigapascals or 350 million times atmospheric pressure.
All this time, the drill is pulled towards the core by the Earth’s gravity. At the center of the core, gravity would be similar to being in orbit and essentially weightless. The reason is that the earth’s mass pull will be equal in all directions. Then, as the drill moves to the other side of the planet, the gravitational pull will change relative to the drill’s position, pulling it back toward the core.
As the drill returns to the surface, it must work against gravity and travel back through the outer core, mantle, and crust. Barring all obstacles, when you get to the middle of the earth, the biggest problem is that you have a long way to go to get to the other side of the earth.
