Tip of the Iceberg
I am quite sure that you have probably heard the idiom, “tip of the iceberg,” at least once in your life. This expression basically means an event that is merely a part of a larger and more complex (often a dark) issue. Over 90% of the volume of iceberg is submerged; therefore, it is always very hard to guess the size of the underwater portion.
While it is difficult to identify the exact origin of this expression, the first usage was officially recorded in 1969. Since RMS Titanic sinking happened 57 years before the first usage of expression, some people believe that the idiom was inspired by this accident. One of the similar expressions to “tip of the iceberg” would be “does not even scratch the surface.”