The web site supports viewing lists of observations by radius like this:
However, there is no way to do something like this with the user interface; you have to construct a URL by hand.
Make this much easier for users by adding lat, long, radius to the search options on the observations page. This could be instead of, or as an alternative to, the current SW lat, SW long, NE lat, NE long. It would allow someone to easily refine or broaden their search area by just changing the radius and clicking search again.