This man-made channel was blasted out by German miners, in the early 1900s, to allow their boats easy access from the inner lagoon to the open sea. Over the years, it has become much narrower as coral formations have developed and now live-aboard vessels often have difficulty navigating it unless the tide is right.
It is at German Channel Wall, at the entrance, that the diving takes place. There is a sandy bottom at 17m that slopes away steeply to the depths. The bottom has some large coral formations. The biggest attraction here is the large number of manta rays you're likely to see - only in Yap will you come across more. They come in on the tide to reach the lagoon and make good use of the resident wrasse to cleanse themselves of parasites.
Grey reef sharks are also often encountered, again taking advantage of the cleaning wrasse. Other attractions are barracuda, lionfish, snappers and triggerfish.