tip of the iceberg

(just) the tip of the iceberg (a problem or difficult situation that shows that a much more serious problem exists; nly a small part of a much larger problem — верхушка айсберга
(just) the tip of it ≅ это еще только цветочки

Example 1: The recent riots are just the tip of the iceberg. (MM)

Example 2: You know, what happened last night,
that's just the tip of it.
(The Deep End of the Ocean, Mv)

Example 3: This figure represents only the tip of the iceberg, since as many as 90% of cases go unreported. (OALD)

See also
[thin edge of the wedge]