Just take a look through the forums here for my post about my trip up to New Hampshire. I went up to a cabin on a lake in NH and took some sodium metal with me. Took a few pictures of the explosions. When you throw a nice sized lump into water, there is a substantial delay as the metal reacts with the water. You hear a lot of hissing and fizzing and you start to get kind of nervous because you know it's going to explode, but you don't know when. When it finally does detonate, it's like the sound of a 12-gauge shotgun going off and a bright orange fireball goes up in the air followed by a mushroom cloud of NaOH smoke. If the metal hasn't completely disintegrated from the heat of the explosion, small little chunks of Na will fall down onto the water and fizz and pop for a while. The explosion is VERY unpredictable which is what makes it a bit dangerous.