Anyone know how to do the 02 random bubble effect?

Hi all, can anyone provide me with some tips or links to create something similar to 02s bubble effect, I have a few ideas but a start would be to actually understand how to do it.

Its not the look of the bubble that concerns me, its the actually randomness of the effect from a location, I am sure I seen a tutorial for this somewhere but for the life of me cannot remember.

Any help more than appreciated.

Thanks

T