Game worlds are typically quite static, with some games allowing you to change the world, and even fewer allowing changes that affect movement in significant ways. One of the ideas I wanted to explore was to have the world change significantly, but the coordinate system keep the player in a “reasonable” place even while the world was changing under him. The game idea is that you are playing the role of a tiny ship inside a giant creature. As you traverse the blood vessels and shoot the white blood cells of the immune system, the creature becomes more agitated. As its heart beats faster, ripples run down the arteries, and the entire creature may start moving about. The game becomes more difficult not because the enemies are tougher, but because the world itself is changing and becoming somewhat disorienting.
Turn on animation to see some movement of the map.
The article about the demo goes into more detail about the code.