Apparently someone rediscovered the fact that when you drop candy into soda, you can get a geyser effect, and it's been making the rounds on the internet (see this video or this video). It was even featured on NPR.

People are billing it as an "explosion" but it's really not. Wikipedia claims that it's a result of adding gum arabic and lowering surface tension of the soda, but that doesn't wash with me - lots of sodas already have gum arabic, and the amount you're adding is going to be minimal (and in solid form). In fact, if anything, since gum arabic is a mixture of carbohydrates and proteins, I would actually expect that it would increase surface tension slightly. What's probably going on is that by dropping candies (Mentos, in this case - I used Nerds when I was a kid) into a bottle of fresh soda, you suddenly and dramatically increase the number of potential nucleation points for the carbon dioxide in solution to come out of solution as bubbles. Those bubbles of gas take up more space than dissolved gas would (example: when divers get "the bends") causing the liquid to be ejected forcefully from the neck of the bottle.