The tip of the iceberg
Definition 1 of 1
Pro Tip 1/3Metaphorical Use
This phrase is used metaphorically to suggest that more issues lie below the surface, unseen. 
The employee's complaints were the tip of the iceberg, indicating deeper workplace problems.
Pro Tip 2/3Hidden Magnitude
It implies a problem or situation is much larger than it appears, with 'the iceberg' being mostly beneath the water's surface. 
The budget discrepancies at work were just the tip of the iceberg for the company's financial issues.
Pro Tip 3/3Conversation Starter
Use this phrase to introduce a topic that has underlying complexities you want to discuss. 
His tardiness was the tip of the iceberg; let's talk about his overall performance.