I'm not quite sure if this is the result of a mod or not, but there is an opening in a section of Whiterun's city wall that if I walk though the opening it leads to a low texture landscape.

screenshot:

http://steamcommunity.com/profiles/76561198112330024/screenshots/?appid=72850&sort=newestfirst&browsefilter=myfiles&view=imagewall&privacy=8