The interior of the Earth, similar to the other rocky or terrestrial planets, is divided into layers. The mantle is a highly viscous layer between the crust and the outer core. Earth's mantle is a rocky shell about 1,800 miles thick that constitutes over 80% percent of the Earth's volume (The part of the Earth best known to us humans.). Two thousand miles beneath our feet, the Earth's solid rock – known as the mantle – gives way to the swirling liquid iron of the outer core. The last few hundred miles of the lowermost mantle is also known as D” (pronounced dee-double-prime). D" is one of the most enigmatic parts of the Earth which scientists have struggled to understand for decades; it can only be measured remotely, using seismic waves from earthquakes.