What I find interesting is that I have the same hole, well, mine is a little different. It only contains half of the lake and nothing around it as you can see here.

This is strange is because I have a scenery program that has more accurate elevation and landclass data for Flight Simulator. If this were a problem with the terrain mesh, the program would have fixed it. My guess is it's a problem with the lake's elevation. Someone at Microsoft messed up half the lake by over 15 thousand feet. Oh well, it makes for an interesting flying experience. Kirk's F-16 seems to handle very well in there---I was able to fly from the bottom to the top, at a 90