Have you heard the phrase tip of the iceberg? It means you are only seeing a tiny fraction of something. An iceberg is a large piece of ice that is floating in a sea/ocean.
Only 10% of the iceberg is visible above water. The remaining 90% is actually under water.
That's why the phrase is used. Just like you can only see 10% of an iceberg, the phrase means you are seeing only a tiny fraction of something. But, why is only 10% of the ice above the water? Why doesn't the ice fully sink, or fully float? Let's find out!