Cool -- I vividly remember my High School Chemistry class demonstration on this, with a pea-sized bit zipping around the surface of the water.

I want to see a big brick of it (1kg or so) in a similarly breakable but enclosing container and held 10m or so underwater in a lake (or something) by a wire mesh cage. Would chopping it up into smaller pieces to maximize the surface area increase the effect? Or would the violence of the reaction make cavitation / hydrogen bubbles that push the water out of way and make the reaction happen in multiple phases as the water gets pushed away and returns?

I can, almost, oblige... watch this *related=http://videosift.com/video/WWII-Newsreel-of-Exploding-Sodium video. The first barrel is close to what you're after.

WWII Newsreel of Exploding Sodium has been added as a related post - related requested by oritteropo.

Awesome! Thanks for the link, I had missed that one.

About the only way to improve on that would be if Mythbusters or somebody did a large amount sliced thin to maximize surface area, built a cage sturdy enough to keep it submerged, and then filmed it (underwater) in ultra high-speed...

Moving this video to oritteropo's personal queue. It failed to receive enough votes to get sifted up to the front page within 2 days.

*discard

Discarding this post - discard requested by original submitter oritteropo.