There is no resistance, so currents will flow indefinitely. The cause is lowering the superconductor below its critical temperature (T_c). Usually liquid helium or nitrogen is used to cool a superconductor below T_c.
A nice explanation by nobel laureate Leggett can be found here:
http://online.physics.uiuc.edu/courses/phys598sc/fall08/L14.pdf
Very readable is also the article by Brun Hansen, "Infinite conductivity of ordinary and gapless superconductors", Physica, Vol 39, p. 271-292, (1968)