Meaning:

Something that shows or suggests a much larger problem.

If you say "It is just the tip of the iceberg," you mean that what you see is just a small part of something that is actually much bigger than it seems.

Example:

I heard 4 government officials were arrested for bribery yesterday.
It's just the tip of the iceberg. Many more are doing the same thing.

The reporter said today's death toll might be just the tip of the iceberg.

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