The center of the Earth is very hot through Blank 1 Question 2 decay, and remaining heat from Earth's formation. Blank 2 Question 2 water reservoir are kept warm through this heat. Blank 3 Question 2 energy is the extraction of this heat energy either to use as heat, or to generate electricity. Water is pumped up from these reservoirs and used for heating. Alternatively, the water can be used to produce steam and spin a Blank 4 Question 2 , creating electricity. This type of energy is very reliable, and consistent, but is location dependent (needing Blank 5 Question 2 or tectonic hotspots) and has very high start up costs