The tip of the iceberg

The idiom the tip of the iceberg means a small, visible part of a much larger problem or situation that is mostly hidden. What people can see or know at first is only a tiny part of the whole.

The expression comes from real icebergs at sea. Only a small part of an iceberg is visible above the water, while most of it is hidden below the surface.

The missing money was only the tip of the iceberg once the full investigation began.