The repeated detonation of nuclear devices underground in salt domes, in a somewhat analogous manner to the explosions that power a car’s internal combustion engine (in that it would be a heat engine), has also been proposed as a means of fusion power in what is termed PACER. Other investigated uses for low-yield peaceful nuclear explosions were underground detonations to stimulate, by a process analogous to fracking, the flow of petroleum and natural gas in tight formations; this was developed most in the Soviet Union, with an increase in the production of many well heads being reported.