1
general::
Phrase(s): the tip of the iceberg
Fig. only the part of something that can be easily observed, but not the rest of it, which is hidden. (Referring to the fact that the majority of an iceberg is below the surface of the water.) • The problems that you see here now are just the tip of the iceberg. There are numerous disasters waiting to happen.
McGrawhill's American Idioms And Phrasal Verbs
2
general::
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. [Mid1900s]
American Heritage Idioms