In the Shadows and Shaders update, coming soon, all maps will be removed to make way for the update, and the default Slate maps will be re-done.
Slate is a default map that takes place on a dark, reflective surface with a deep blue skybox.
Skylands is a default map that takes place on a bright, reflective surface with a sunny daytime skybox, much like the original slate.
Slate Sea, known ingame as Slate Sea Revised, is a Slate map that is flooded with water. It has a late afternoon skybox, and comes with a variation with a darker, cloudy skybox and a constant rain emitter beginning in the sky.