But unlike the Titanic, icebergs are good for writers ;). As long as we use them properly.
A reader really only sees the surface of any story. As readers, we may see depths that are hinted at, or events that we knew influenced who the character is, but we don't see the whole thing. Readers see the tip of the iceberg.
But as writers, we NEED to know the entire iceberg--the huge back story, what motivated the characters to do what they do, world-building, in some cases, universe-building that can cover everything from history to religions to the way magic works in our worlds.
But we shouldn't show it.
I know, I just said it has to be there, and the reader needs to feel it's there, feel the weight of the history of the world you've created (even if you have a contemporary story), but you can't put it in your book.
It can be painful sometimes, you do a ton of research on historical events, weapons, religion, whathaveyou, create a fully functioning world, only to be told you really shouldn't share it with readers.
Because readers don't care.
All of us pick up a novel because we want to get lost in the story. And as awesome as it is, research, history, and world-building are NOT story. You may have spent hours tracking down the perfect period authentic dagger for your Mongolian romance from the 1600's. You may know all about it, how it was made, the ceremonies behind it, etc. And all of that for it to just pop up as a piece of set-dressing. You cannot devote a page or two to all of the wonderful back-story of that dagger. Your reader just wants to know who is using it and who its been stuck into recently.
The problem with some authors is that they have all this cool stuff--and they want to USE all that cool stuff! "I know this! It's awesome! I spent years developing my world and want everyone to know the smallest detail!" Those bits, the under-the-water side of your iceberg, are not story. They support your story, give it a weight and heft it would lack otherwise (had the iceberg that took down the Titanic just been some floating surface ice, they probably wouldn't have gone down) BUT they are NOT your story.
So keep building your icebergs, you need them for a good story, but remember that the strength of the iceberg isn't what's seen on the surface.