Ground source heat pumps use something called a ground loop to operate. Ground loops are buried deep in the garden and the depth is dependant on how much room you have.
The length of loop is intrinsically linked to its heat output. The longer the loop, the more energy it will produce. Typically, loops are laid flat or coiled in trenches around two metres in depth. However, houses with little garden space can have the loop buried vertically.
The ground loop contains a mixture of water and antifreeze which circulates through constantly. The heat from the ground is absorbed into this fluid and is then pumped through a heat exchanger.
Low grade heat is concentrated into a higher temperature so it is capable of heating a home or water supply. Once it has done its job the fluid is cool again and goes back into the ground, and the process continues in this cycle.
Because the heat pump needs electricity to run, it does have some impact on the environment, but the heat it takes from the ground is completely renewable.
Heat pumps do deliver heat at a lower temperature than gas or oil boilers, but can provide heat energy over a longer period.