This is a new phenomenon that you can observer in all of its (un)scientific glory at YouTube or Google Video. It appears that when you mix Mentos with diet Coke you get a geyser.
What was the brand manager’s reaction? “It’s an entertaining phenomenon … [but] doesn’t fit with the brand personality of Diet Coke”… Indeed.
The reason I chose this specific video is because of Steve Ballmer’s Developers remix. There’s a ton of others – DIY “how to mix mentos and coke” videos.