This cavern is drastically different from other caverns, as it's laid out vertically instead of horizontally. The Redoubts and AT Plants are on the floor, while the Module Buildings are near the top. The Core Complex lies right in the middle. Buildings and rock seem to hang in mid-air.
Many inexperienced cavers find it difficult to find their way to the top of the cavern from the bottom. This video shows some of the most common and efficient ziplines to navigate the cavern.
It can be assumed that the origin of the cavern's name comes from the paradise-esque underworld Annwn in Welsh mythology.