Laeradborg Reef
This huge barrier reef is the home of a variety of sea life, but deadly to ships of all kinds, or at least the large ones, as there are some small hidden gaps that something like a canoe, or a dolphin might go through. In addition to the reef itself there are numerous waterspouts (tornadoes that suck up water) that constantly circle the reef making air travel over the reef virtually impossible (as well as walking over it). For those that think swimming through the gaps would be easier, they have to contend with razor sharp coral and the sometimes dangerous sea life that makes the coral it's home. The reef is not completely impassible, but extremely difficult, especially without help from inside. Thus only those who really want to may pass to see the Mediator in the World Tree. 
