Inspired by Nelson Minar's map of US rivers, Mike Bostock demonstrates
how to generate your own TopoJSON from the same river data. As
indicated by the name, the file format is a way to encode topology,
and it does so in a compact way.
Casey Thomas then forked Mike's code to make TopoJSON from TIGER/Line
files, which is essentially data supplied by the Census Bureau that
represents the roads in the United States.