As mass goes faster and faster the energy being put into making the object go faster gets converted to mass according to the famous equation E = mc2. When a proton speeds up in an accelerator its mass increases. As a  proton of mass 1 accelerates to a speed of 99.9997 the speed of light the mass ends up 430 times the mass of the original proton.

(Book – E=mc2: a biography of the worlds most famous equation : David Bodanis)