The Tropics biome is a new biome that will be generated in your world upon entering Superhardmode. It will overtake the ocean on one side of your map, and in it will spawn Man of War and Salmon NPCs. This biome is made up of Black Sand tiles, Tropical Mud and Grass, and Tropic Stone.
If you enter the Tropics, you'll get an achievement: 'Jungle!... not?'