Take a tour to under the earth,

Journey to center of the earth

The Earth's interior is composed of four layers, three solid and one liquid—not magma but molten metal, nearly as hot as the surface of the sun. 

The deepest layer is a solid iron ball, about 1,500 miles (2,400 kilometers) in diameter. Although this inner core is white hot, the pressure is so high the iron cannot melt. 

The iron isn't pure—scientists believe it contains sulfur and nickel, plus smaller amounts of other elements. Estimates of its temperature vary, but it is probably somewhere between 9,000 and 13,000 degrees Fahrenheit (5,000 and 7,000 degrees Celsius). 

Above the inner core is the outer core, a shell of liquid iron. This layer is cooler but still very hot, perhaps 7,200 to 9,000 degrees Fahrenheit (4,000 to 5,000 degrees Celsius). It too is composed mostly of iron, plus substantial amounts of sulfur and nickel. It creates the Earth's magnetic field and is about 1,400 miles (2,300 kilometers) thick. 

River of Rock of earth,

The next layer is the mantle. Many people think of this as lava, but it's actually rock. The rock is so hot, however, that it flows under pressure, like road tar. This creates very slow-moving currents as hot rock rises from the depths and cooler rock descends. 

The mantle is about 1,800 miles (2,900 kilometers) thick and appears to be divided into two layers: the upper mantle and the lower mantle. The boundary between the two lies about 465 miles (750 kilometers) beneath the Earth's surface. 

The crust is the outermost layer of the Earth. It is the familiar landscape on which we live: rocks, soil, and seabed. It ranges from about five miles (eight kilometers) thick beneath the oceans to an average of 25 miles (40 kilometers) thick beneath the continents. 

Most major depressions are associated with tectonic plate boundaries. They form when converging plates deform or when spreading centers open. A few are volcanic in origin. Most of these depressions are in the northern hemisphere for a good reason: this is where most of the land is and where most of the plate boundaries occur. Many are found in the vicinity of where the African, Arabian and Eurasian plates meet. Most are found in hot desert regions of the planet where high evaporation rates prevent them from filling with water. A few are found in temperature climes.

Many well-populated places in the world are below sea level. About a third of the Netherlands including Schiphol Airport is below sea level. So too are the Jordan River and parts of many coastal cities including New Orleans and Bangkok. Other regions below sea level are the Enriquillo-Cul de Sac Depression (Hispaniola), Chott Melrhir Depression (Algeria), Sebkhet Shunayn and Sabkhat Ghuzayyil (Libya), Shatt al Gharsah (Tunisia), Lammefjord (Denmark), Sebkha Tah (Morocco), Hachiro-gata (Japan), Sariqarnish Kuli (Uzbekistan), The Fens (UK) and Lake Eyre, Australia.

Currents within the mantle have broken the crust into blocks, called plates, which slowly move around, colliding to build mountains or rifting apart to form new seafloor. 

Continents are composed of relatively light blocks that float high on the mantle, like gigantic, slow-moving icebergs. Seafloor is made of a denser rock called basalt, which presses deeper into the mantle, producing basins that can fill with water. 

Except in the crust, the interior of the Earth cannot be studied by drilling holes to take samples. Instead, scientists map the interior by watching how seismic waves from earthquakes are bent, reflected, sped up, or delayed by the various layers.