A tree melts into the ground after listening to loud music in the hot sun, so you have to find a nice big tank to scoop the melted tree into. The music made the tree vibrate at juuust the right frequency that it, combined with the hot sun, made the tree turn into a puddle. You can't just leave it being all blobby on the ground, so you find a tank for it.
You're staring at the big tank when an eerie sound comes out, "Oooh I am the soul (そう) of the tree! You've trapped me in this tank and now I'm going to haunt it!" You don't want to be haunted by a tank with a soul stuck in it.