This is an old hack that's been around for a while;

http://planetelderscrolls.gamespy.com/View.php?view=OblivionMods.Detail&id=63

I played with it briefly when I first got Oblivion, but in all the wrong places like south of Skingrad, northwest of Chorrol, and other areas where the mountains were too high to make terrain editing easy. Recently I noticed a lot of empty area outside the narrow borders of Leyawiin county where they didn't go psycho with steep mountains. So I blew the dust and cobwebs off the old hack and tried adding some markers to see how far east I could go. The terrain stops with an "edge of the world" way east of Fort Blueblood, but the ingame map doesn't go all the way to the edge, so after some experimenting with map markers I flattened an area and started building a city about 15 cells east of Fort Blueblood.

https://scontent-b.xx.fbcdn.net/hphotos-prn2/t1.0-9/1554393_726490200728596_742750914141061999_n.jpg

Originally had the walls over to the right of that green grass painted strip, but testing in game the terrain got weird so I moved it west.

https://scontent-b.xx.fbcdn.net/hphotos-prn2/v/t1.0-9/10256131_726490134061936_5589439538649046025_n.jpg?oh=c07edfdc52a9000bc5b992d53d7794b0&oe=53BD505D

Pretty sure this is something to do with lowering the terrain, because I can go quite a ways further east in game on the original height terrain;

https://scontent-b.xx.fbcdn.net/hphotos-prn2/t1.0-9/10157337_726490177395265_272637323175582067_n.jpg

Lot of questions to come, but the first one is, did they set some kind of minimum height when they created the original terrain, or is there something special about the cells with the green grass strips? Like region borders? This one extends north and south a long way, but it's well past the actual game borders and well inside the right edge of the ingame map.