"bubble mask" effect

[font=verdana, arial, helvetica][size=2]I’ve been seeing a particular effect on a lot of Flash sites, and would like to do something like it. You can see it in the intro of this site http://lifeaquatic.movies.go.com/main.html, its a sort of mask transition, with these circles growing sequentially to reveal the image beneath. Is it action script? A plug in? Voodoo? Hard work? Any help would be great, thanks![/size][/font]