If you plant a letterbox somewhere in a county, state, or country, and that area is the 'mystery area', just type the area like you would any other location.

Often times, the geocoders might support more non-standard areas such as "Oregon Coast" or "San Francisco Bay Area." If the geocoder cannot find your non-standard area, however, you can still create one yourself by following these steps for a custom location. Let's assume the geocoders cannot figure out what you mean by "Oregon Coast," so we'll have to create our own custom mystery area. Follow these steps:

1. Fill in the name of the location with the label we wish to use: Oregon Coast.
2. Leave the address blank. The Oregon Coast does not have an address!
3. Leave the city blank. The Oregon Coast is not contained within the boundaries of a single city.
4. Leave the county blank. The Oregon Coast is not contained entirely within a single county.
5. Fill in the state as "Oregon." Yes, the Oregon Coast is completely contained within a single state—the state of Oregon, and AQ needs to know this.
6. Fill in the country as "United States." Again, the Oregon Coast is completely contained within the United States, so we can (and should!) include the country.
7. Pull up a map of Oregon and eyeball the location that, roughly speaking, marks the center point of the Oregon Coast. If you need help with this, check out How do I find a GPS coordinate for a location? In this case, eyeballing the center point of the Oregon Coast, we determine it's coordinates to be 44.347422, -122.645874. Enter that as the coordinates.
8. Now eyeball how far it is from that center point to the edge of the area to be covered—in this case, the entire distance from the Washington border to the north to the California border on the south. The entire distance, north to south, is about 300 miles as the crow flies, and half the distance (near the center point), would be about 150 miles. Enter 150 as the radius.
9. And save your changes! AQ now supports mystery locations for boxes "somewhere on the Oregon Coast."

Mystery areas can be of any size anywhere in the world. A mystery location for "Western Europe" that spans multiple countries would require that you leave the state and country blank (since it's not contained within any one state or country), but everything else works exactly the same. A mystery location for "Eastern King County" in Washington state would require that you fill in the county as "King County" since Eastern King County is contained completely within the one county, but everything else works the same.

You could even mark a mystery area that includes only part of a city, such as "Southwest Seattle." Since southwestern Seattle is contained completely within King County and the city of Seattle, you'll want to include the county and city names in their respective boxes.

And, for the particularly devious and creative people, you could even "invent" imaginary mystery areas such as "Narnia." Let's say you planted a letterbox and want it's location to be "somewhere in Narnia"—an area that you arbitrarily decide covers the entire mid-west of the United States—fill in Narnia as the name of the location, the United States as the country, and leave the city, county, and states options empty. (You could even leave the country blank if you do not want people to know that Narnia is within the United States.)