This is the Pangaea, built by the UN as a refuge after the earth becomes uninhabitable. it is meant to carry as much of humanity as possible in search of a new home. So far just the hull plating for the top and sides will begin on the bottom soon. Due to its enormous size it was assembled in space and won’t land on most planets because of the sheer power it would take to get it airborne let alone reach escape velocity.
Very cool, something that might sell the size of it is if there is something in the picture that you know the size of and can compare the ship too, visible windows for example.