“Float” is the subject of the sentence. The subject of a sentence is the word that is doing the action. Let’s see how this is determined by first looking for the action of the sentence, which is “collapsed.” Now, we must ask ourselves what was doing the collapsing. The answer to that might be tricky as one might be included to say “the winning float.” However, when we look a little close, we can see that “the” and “winning” are really adjectives modifying the word “float.” As such, it can be determined that only the one word “float” is the doer of the action and, therefore, the subject.