Superficial evidence of a much larger problem, as in Laying off a hundred workers is only the tip of the iceberg. This idiom alludes to the structure of an iceberg, most of whose bulk lies underwater. [Mid-1900s]

Only a hint or suggestion of a much larger or more complex issue or problem: “The money missing from petty cash was only the tip of the iceberg of financial mismanagement.” This phrase alludes to the fact that the bulk of a floating iceberg is concealed beneath the water, leaving only a small portion, its tip, visible above.