"Tip of the Iceberg"
I saw a lot of icebergs in my travels, but this was one of the few where you could see a little bit of what is usually hidden under water.
From Wiki: Because the density of pure ice is about 920 kg/m³, and that of seawater about 1025 kg/m³, typically only one-tenth of the volume of an iceberg is above water. The shape of the underwater portion can be difficult to judge by looking at the portion above the surface. This has led to the expression “tip of the iceberg”, for a problem or difficulty that is only a small manifestation of a larger problem.