Ever since the Titanic, the dangers of icebergs are well-known. The danger is one of appearances; that which appears above the surface seems innocuous enough, but in truth, is only a fraction of the whole which one is about to confront.
It is because of this that the iceberg has become a metaphor for anything which lies mostly hidden from view, beneath the surface. For example, the psychoanalytic model of the mind views consciousness as the fraction that appears above the surface, and the subconscious as the majority which lurks beneath.
Another example is one that I’m fond of from Yeshiva days. Every once in a while, there will be a Tosafot which begins at the bottom of an Amud Aleph and goes for a few lines until the end of the page. Since it’s rare for a Tosafot to extend from one page to the next, you think it’s a short Tosafot, but then BAM, you turn the page and the thing is a monster, covering the entire next page. That’s an Iceberg Tosafos.
Now, at a later stage in life, I’ve chanced upon yet another type of ‘iceberg’. Parents are responsible for maintaining their kids’ hygiene. That includes noses. So whenever the kids’ noses look stuffed, it’s my job to clean it out. If what needs cleaning is already sticking out of the nose a bit, then I’ll just as often use my fingers to pull it free. Every once in a while, it looks like just a bit of dry mucus that needs to be removed, but when you start tugging, it turns out to be the mother lode. Ive begun calling those ‘iceberg boogers’. The ADDeRebbetzin is not amused.