The small-world phenomenon is the idea that ANYONE on Earth can be connected to anyone else (even michael jackson in his castle 🙂 through just 6 degrees of separation.
This notion has been tested in the late 60s by social psyhcologist Stanley Milgram. His conclusion was that people who seem very distant in physical or social space may actually be closer than we think.
A book about that : Duncan Watts : Six Degrees : The Science of Connected Age
The small-world phenomenon also shows up in neural networks, power grids, biomechnical reactions…