Along with hydropower and fossil fuels, like crude oil and coal, the Earth has other resources of energy as well. These include the geothermal heat which comes from the depths of the Earth. The heat of the Earth normally comes out on the surface in destructive ways in many volcanic regions, and at other places such as Iceland this heat is released through ‘quiet volcanism’. High temperatures prevail deep inside the Earth’s surface, which can be used to produce energy with the help of geothermal power plants. Energy can also be obtained from plants in the form of biogas or fuel from sugarcane, which is used to run a number of cars in countries like Brazil.
Leaving aside the volcanic areas, temperature in the upper layers of the Earth’s crust varies from 10 to 30°C. This weak geothermal energy can still be used by enriching it a little with heat pumps. In the naturally warm sources, the thermal sources, the heat energy of the Earth is used for medicinal purposes – as done by the ancient Romans. 
This happens in a natural way in dung heaps, as also in compost plants, garbage dumps, and crops. In large biogas plants, the remains of the plants are decomposed by bacteria in an air-tight tank. The released gas is highly combustible because of its high methane content. The Earth has almost unlimited supply of plant remains, which makes biogas an important source of energy from renewable raw materials. It can be used as gas for heating or for generating electricity. It can also be liquefied easily and used as fuel for vehicles.
Glass mainly consists of lime, soda, and quartz sand. These three components are milled together finely, mixed with one another, and melted at about 1600°C in a furnace. When the molten glass is cooled to l000°C, it can be shaped into different objects. Depending on the intended use, the glass objects are shaped by pressing or drawing the molten glass. The used glass that we collect is also a valuable raw material, as it can be processed to make new glass objects. 
Diamonds are very expensive because they occur at relatively few places in the world – like Russia and South Africa – and are very difficult to extract. They are made of carbon and are the hardest substance known to us, which is the result of the high pressure at their place of origin deep below the Earth’s surface. They are naturally thrown up on the Earth’s surface through volcanic eruptions. For this reason they are mined in the vents of the extinct volcanoes. Being very hard they have great industrial use for grinding tools. Diamonds can also be made artificially by creating the same conditions as found under the surface of the Earth, but these artificially made diamonds are not as valuable as natural diamonds. 
Windmills are machines that are driven by wind. Their blades have slightly diagonal surfaces. If the wind hits them from front, they deflect it to the side and in this way convert the wind pressure in a rotary movement. Initially, millers had to set the blades themselves in such a way that the wind came onto them from the front. The rotary movement of the blades gets transferred to the milling stones inside the mill through cogwheels. Windmills were traditionally used to mill corn to make flour and grate spices. They also acted as drive for oil presses and sawmills. 
