Move and Repeat Terrain

You can duplicate the terrain then disconnect the terrain when one goes out of view and move this same terrain down (also out of view) and reconnect.

Create a prefab with the terrain in it and a rigidbody attached. Remove gravity. Create a cube and set it to a trigger. (1,1,1) Scale is fine.

Position your camera, You will have to tweak this so you don't see the ground plane spawning.

Move the cube behind your camera. Make sure the cube is level with the ground plane so in my tests both were Y = 0. Mine is at 256,0,-245.

Create an empty object. I placed mine at 0,0,0 my terrain was set to 512 by 512 so I put my camera at 256,5,0. Half way between the terrain.

Create a javascript and attach it to the cube you created and enter this code.

  // Terrain Prefab
  var terrain : GameObject;
  //Spawn Point
  var spawn : GameObject;
  //Speed at which the ground mvoes
  var power : float = 1000;
 
 // calls the spawnTerrain function and spawns a terrain when the game starts
 function Start () {
     
     spawnTerrain();
     
 }
 
 //Crates the terrain from the prefab and applies movement.
 function spawnTerrain ()
 {
     var spawn = Instantiate(terrain, spawn.transform.position, spawn.transform.rotation);
     spawn.rigidbody.AddForce(Vector3(0,0,-power));
 }
 
 // when the terrain collides with the cube a new one is spawned and the current one is destroyed
 function OnTriggerEnter (other : Collider)
 {
     Debug.Log("Spawn");
     Destroy(other.gameObject,2);
     spawnTerrain();
     
 }