It all started simple enough. "Let's put a cache somewhere we can observe it occasionally." Then it became "Let's fill in the gaps to create an interesting caching walk." Now there's more extending further around the "block." They're primarily nanos or micros, often camouflaged, sometimes in larger hosts. Muggle awareness may be required. As always, awareness of things that poke, bite, scratch, or cut and/or could be poison is essential.
Recommend walking or biking the series.