And I thought I had too much time on my hands...

Evidentally if you drop a Mentos into Diet Pepsi it causes a violent reaction akin to vinegar and baking soda; but unlike vinegar and baking soda the reaction is not chemically based. Instead the reaction is the result of carbonation and nucleation sites on the Mentos mint itself. Not to be outdone by some sixth grader's vinegar volcano, these guys at EepyBird.com construct a Diet Pepsi version of the Bellagio Fountains in Las Vegas.