Here, water flows from a hot spring at the Roman site at Bath, south-west England.
The water at Bath rises to the surface at a temperature of at least 45° C.
Some of the heat comes from the core.
The molten iron outer core (red) is slowly crystallising, adding to the solid inner core (orange).
This releases 'latent heat' which flows through the Earth.
However, radioactive decay of unstable isotopes of three elements (thorium, uranium and potassium) produce most of the Earth's internal heat.
The heat flow out of the crust is highest at the spreading ocean ridges.
Even there, it is only around 0.35 watts per square metre.