In many regions of the world, in particular those around tectonic plate boundaries where volcanoes are prevalent, the ground is hot because of the presence of magma much closer to the earth's surface than in other locations. This heat is put to good use in many different ways, such as cooking meat on a BBQ at Timanfaya Park in Lanzarote. As well as humans, other creatures also harness the heat energy from just below the surface for their own needs, like Scrubfowl birds who bury their eggs in the warm ground rather than have to constantly sit on them to keep them warm like other species of birds need to do.
Geothermal energy is a renewable form of natural energy which has also been harnessed by humans. The main application is for heating buildings and so negating the need for consuming electricity which is likely to have been generated through the burning of fossil fuels that are highly detrimental for the environment in terms of pollution and harmful gases such as carbon dioxide given off.
Geothermal energy is similar to other forms of renewable power in that there are often high initial costs whilst the system is constructed and installed, but will then provide energy at little to no cost as the input fuel is provided by nature for free. This large initial expense can often be off-putting for many, either because they simply cannot afford the large outlay or because they fail to foresee the long-term, bigger picture cost savings that will come as time goes by. As far as geothermal energy is concerned, the system will require boreholes to be drilled and the installation of pipes underground, along with a pump system to circulate the water around the system which will be warmed by the underground heat.
There may be additional costs in making an existing building's heating system compatible with a geothermal energy heating system. Also, with any equipment and components which are buried either under the ground or underwater, maintenance and repairs can be much more difficult, time consuming and expensive than they would be if they were above ground and much more accessible.
Another issue is that geothermal wells and boreholes will release gases such as methane into the atmosphere which can damage the ozone layer and cause health implications for living creatures down on the earth. However the amount actually released will generally be far lower than emissions from burning fossil fuels, which means that overall geothermal energy is much more preferable for the environment when it comes to energy and power generation than a fossil fuel power station is.