An example of E = mc^2

This picture was taken in the CERN 2-metre hydrogen bubble chamber, filled with hydrogen and exposed to a beam of energy 16 GeV.

In the interaction, a considerable amount of this energy was 'converted’ into mass – the final state has, in addition to a proton and a , 16 extra pions (8 negative and 8 positive to conserve charge).
For another example click here.