Tip of the Iceberg

Icebergs keep roughly 88% of their mass underwater, hidden from view, a striking enough natural fact to become shorthand for anything where the visible portion is only a small fraction of the real scale. The figurative phrase is documented in American use by 1912, and the sense had clearly become familiar enough to use casually by then. Given the timing, right after the Titanic sank in 1912 after striking an iceberg, it's an easy assumption that the disaster inspired the phrase. There's no actual evidence for that connection. The figurative use of "iceberg" for something largely hidden was already established before the ship ever set sail.