The sun is the ultimate source of energy for most of the life on Earth.
But there are many creatures which inhabit areas that receive no sunlight. These organisms use energy that is given off by the Earth instead of energy from sunlight. This energy could be in the form of heat energy which is released from thermal vents, chemical energy that is released by the breaking down of chemicals like hydrogen sulfide or in a few cases even energy released from the decay of uranium.
Some examples of these organisms are members of the D. audaxviator bacterium species. A few members of this species use energy released from the decay of uranium to catalyze all the reactions that form the compounds which it is made up of. Others use heat energy released from thermal vents to power themselves. A few break up the chemical hydrogen sulfide that the water around them is saturated with and use the energy released to survive.