The center of the earth is a very hot place. Jf we could dig down 48 kilometers into the earth, the temperature would be about 1,200 degrees Centigrade. At the core or center of the earth the temperature may be about 5,500 degrees Centigrade. At such temperatures, rock exists in a molten form. Lava is the molten rock, mixed with steam and gas, that is forced out of the interjor of the earth. It comes from the center of the earth through cracks in the solid surface. Sometimes the crack may be a rounded hole. When the lava comes out, it spreads out into a kind of round puddle and cools into rock. If more lava is forced out later, it flows over the first deposit and makes it a little higher. As this continues to happen, layer after layer is built up and there is finally a mountain of rock which we call a volcano. When a flow of lava occurs and spreads over the land, it destroys everything in its path. This is because it is a heavy stream of molten rock with a temperature of 1,090 to 1,640 degrees Centigrade. Cities that are close to volcanoes are always in danger of being destroyed by such a flow of lava. Sometimes a very long period goes by without this happening, and people assume they are now safe forever. Then suddenly the flow of lava begins again. This happened two thousand years ago to a Roman city in Italy, called Pompeii. It was buried completely under the flow of lava from the famous volcano, Mount Vesuvius.