I created a level that incorporates some map terrain created with NEM's and some walls created with WED. When I build the level I get an error:

W092: Level Leaking!

The level is completely surrounded by a sky box, so I don't know why I am getting this error. If I delete lage portions of the terrain the error goes away. This is the first time when using a terrain generated by NEM's that I have encountered this error.

My question: How do I discover exactly WHICH block or blocks is causing this error so I can fix the leak? (other than deleting one block at a time ).