OK being inspired by the Diaper Volcano thread I decided to try something similar, and with better results I may add. I placed Alka-Seltzer in a diaper and added water.
Now the first time I tried this I took the contents of a 36 count pack and put them in a coffee grinder to turn them into a fine powder. It took too long and I believe shortened the reaction time.
Fast forward to just a moment ago I escalated the experiment to opening 72 tablets and pouting them whole into a diaper. I then stepped into the tub and added a half a gallon of water. The result was quite interesting and I believe will be repeated on a larger scale.
Que Mythbusters theme music. Next time, I think I'll try between six and nine boxes, that would be between 216 and 324 tablets.
Also I'm not sure it will work well and am sure the cleanup will be more involved, but I'm thinking of filling a diaper with diet coke and adding mentos.