hey man, I love that game so much especially that effect with the horizon. As I game developer myself I need to know how you did this! Do you mind explaining how you made that effect?
I'm looking at the positions of sprites on the Y axis, and I manipulate them once they cross the horizon. As soon as they do, I take their distance from the horizon and add it to their actual position with an opposite sign. So stuff that should be moving upwards the screen will start moving down the screen once they pass the horizon.
I also use the distance from the horizon to slow down the movement as they get farther so the movement is not linear, which makes the dip feel more like a curved surface instead of an edge.
Hope this helps! Thank you for the comment :)