Forum Moderators: open
In addition - Brett's original post was based on a single domain/site.
The overall concept is based on related information.
You start with a web site about "trees" which once you have covered all things about trees -- you may want to venture expanding your site into:
pulp & paper
lumber mills
hydro-electric power
alternative power resources
the environment
climate change/global warming
weather phenomena
disaster relief
...and so on. The "Pyramid" affect (or the broading of each sub-section as well as the site itself) is endless.
Each new topic is a topic unto itself but also related to the next and/or many adjacent topics. The commonalities between the subject matter allows linking to develop vertically as well as horizontally.
Search Engine Theme Pyramids and Google [webmasterworld.com]