title: What XNA based 3D terrain and physics libraries exist?
body: <p>I'm planning on creating a game that contains a landscape with objects on it. The landscape will be defined using a heightfield, and the objects will move about on top of, and fly over the terrain. If you've ever played the old games <a href="http://en.wikipedia.org/wiki/Marble_Madness" rel="nofollow noreferrer">Marble Madness</a> and <a href="http://en.wikipedia.org/wiki/Zarch" rel="nofollow noreferrer">Virus/Zarch</a>, that's the kind of complexity and style I'm trying to create.</p>
  
  <p>I've seen various physics engines on the Internet, and a few tutorials about how to render heightfields as terrain, but they either lack documentation or seem overly complex for what I need.</p>
  
  <p>All I need is a way to draw a heightfield, place 3D objects on it and then make them roll down the hills, or fly around in the sky. I don't mind making my own code to do this, so any relevant tutorials would be good too.</p>