The Alternate Terrain Generator gives you a new option for generating worlds in Minecraft. It allows for a much more dramatic and realistic world with rolling fields and massive mountains. It gives you a lot more than a normal generation will give without causing significant lag.
Mountains are much steeper and quite daunting. You’ll also come across many mountain-born rivers and streams. This allows for some neat variation in mountainous terrain as well as alternate water sources.
Down the hills you may come across fields of shrubland. It’s a new look for Minecraft with bushes instead of trees.
And you’ll also see swampy coastlines instead of just sand!