Atlantis is a soft, medium blue. A true blue, without teal or green undertones, medium in depth and hue, Atlantis is dark enough to not be a pastel, but light enough that it will show cables and other stitch patterns to great effect.
I love this blue, and named it after the (imaginary?) city of Atlantis because it reminds me of ocean water with just a bit of sunlight streaming in. Like, perhaps, it would look from Atlantis looking up towards the surface.