Thorium is the most energy-dense substance on Earth, and enough exists to power civilization for millennia.
Widespread use of energy has been key to the advancement of human civilization. The largest fraction of the world--the developing nations--aspire to a higher standards of living that will require yet more energy.
All current commercial nuclear power reactors depend primarily on uranium-235 or the resulting reprocessed plutonium as their fuel. Another nuclear fuel is uranium-233 derived from naturally occurring thorium.
Thorium is a slightly radioactive metal in the Earth's crust, four times as plentiful as uranium. In a nuclear reactor thorium-232 is transformed by neutron capture and natural decay into uranium-233, which undergoes fission within the same reactor to provide heat and power.