The boundary between the nations of United States of Mexico and the United States of America, but also the intersections of the first world, the third world, and the world of Neptune. The border fence sadly reminds one of similar boundaries in Berlin and Palestine.
A local gem that is almost literally in my back yard, Mission Trails Park is good hiking and exploring. A couple trips chronicled here from 2011.
At the edge of the earth, on the threshold of the kingdom of Neptune. Or just down at the end of Point Loma, if you're looking for it on a map.