The coastline paradox is the counterintuitive observation that the coastline of a landmass does not have a well-defined length. This results from the fractal curve-like properties of coastlines, i.e., the fact that a coastline typically has a fractal dimension. The first recorded observation of this phenomenon was by Lewis Fry Richardson[1][2] and it was expanded upon by Benoit Mandelbrot

[...] A key property of some fractals is self-similarity; that is, at any scale the same general configuration appears.

https://en.wikipedia.org/wiki/Coastline_paradox