Any of you that follow Bruce Schneier might remember he has these "movie plot" contests, to come up with plausible threats which would incur another bit of security theatre.
TL;DR, take some sodium metal onto a plane, drop it in water. As one of the first comments said, it's not plausible:
> Not plausible. I've seen a half-pound of reagent-quality sodium dropped in a river. The sodium burned violently; the chunk shattered; the expanding gas launched broken bits out of the river, which landed back in the water and kept on burning. Nothing, however, with enough force to make me believe it would cause a full-on explosion.
> Further, sodium oxidizes in air; sodium metal fashioned into any innocuous item and exposed to air (as any innocuous metal item I can think of would be) would have reacted away a great deal of its mass in the air between packing and getting on a plane. There's a reason sodium is normally stored in kerosene or some other liquid it won't react with. Dropping oxidized sodium into water is a far less interesting reaction.
The winner in 2007 was a plot where water would have to be banned... https://www.schneier.com/blog/archives/2007/06/second_moviep...