// look for all ressources in folder Terrain
UnityEngine.Object[] prefabs =  Resources.LoadAll("Terrains");

        for (int i = 0; i < prefabs.Length; i++)
        {
            GameObject terrainGO = new GameObject("My Terrain");
            //Terrain terrain = terrainGO.AddComponent<Terrain>();
            Terrain terrain = (Terrain)terrainGO.AddComponent(typeof(Terrain));
            TerrainData terrainData = new TerrainData();

            terrain.terrainData = terrainData;
            terrain.terrainData.size = terrainSize;
            terrain.terrainData.heightmapResolution = heighMapResolution;

            Instantiate(terrain);

        // set terrain as prefab
            prefabs[i] = terrain;
        }