Atomic nucleus
When two light nuclei come into very close contact with each other it is possible for the strong force to fuse the two together.  It takes a great deal of energy to push the nuclei close enough together for the strong force to have an effect, so the process of nuclear fusion can only take place at very high temperatures or high densities.  Once the nuclei are close enough together the strong force overcomes their electromagnetic repulsion and squishes them into a new nucleus.  A very large amount of energy is released when light nuclei fuse together because the binding energy per nucleon increases with mass number up until nickel-62.  Stars like our sun are powered by the fusion of four protons into a helium nucleus, two positrons, and two neutrinos.  The uncontrolled fusion of hydrogen into helium is known as thermonuclear runaway. Research to find an economically viable method of using energy from a controlled fusion reaction is currently being undertaken by various research establishments (see JET and ITER).