The reviewers use a rule of thumb that caches placed within .10 miles (528 feet or 161 metres) of another cache may not be published on the site. This is an arbitrary distance and is just a guideline, but the ultimate goal is to reduce the number of caches hidden in a particular area and to reduce confusion that might otherwise result when one cache is found while looking for another. On the same note, don't go cache crazy and hide a cache every 600 feet just because you can. If you want to create a series of caches (sometimes called a “Power Trail”), the reviewer may require you to create a multi-cache, if the waypoints are close together. A series of caches that are generally intended to be found as a group are good candidates for submission as a single multicache.
We didn't have any power trails in Iowa until a few weeks ago when I approved the LOBRI Trails in Muscatine, after geocachers LOBRI wrote to me to request that their series be approved so they could "draw attention to geocaching" in Muscatine in advance of an upcoming geocaching event and so they could take a local newspaper reporter on a cache hunt. Even though Grounspeak discourages power trails (see the guideline excerpt above) I decided to approve this series. Evidently that power trail or others outside of Iowa inspired other geocachers to create their own power trails. Now several more have been submitted.
Because of what's written in the guidelines, I'm going to begin questioning geocachers who submit power trails -- asking why they can't be set up as multi-caches instead. If there is no good reason, they can expect that their power trail cache will not be approved.