After being informed of a possible landscape rip, I wanted to inspect this and see how bad it was if indeed it was a landscape rip. It turned out to be east of Windhelm and was viewable from the MAP just over a small island. After checking it out, I discovered no rip from looking down at it from high "tcl". So I thought I might see it from underneath and went below the plane. I found that it was at a 32x32 transition and the rips were indeed rips but they were textured and dropping downwards so that you wouldn't have to see the "empty" space from the non-connected vertices. Okay, so that's cool. Looking around then, I noticed the plane was extremely low textured and was wondering why. Looking up to go back to the topside of the island, I noticed there is another plane that was the high textured version you play in. I checked this out around White Run too, and it had it. So, the game has two planes? FO3 didn't have this, I think, I don't remember, but Oblivion didn't have this for sure. So what's up with this? What's the purpose?

It's a safety ground, so just in case you fall through the ground, the safety ground catches you so you don't fall into the bottomless pit of nothing-being-there.

Or maybe not...

Possibly, in the Riften 'hall of the dead' there's a missing floor tile... when I jump down the hole it pops me back up into the room (obviously I wouldn't recommend doing this anyway!). [or at least, there's a missing floor tile for me].

On the other hand, if the lower terrain is textured, that would be weird.

Oblivion, fallout 3 and fallout new vegas all have that extra plane below the one you actually use to walk around on, don't ask me why, tho.