Hi Robert, it looks like the problem is that not all the edges on the bottom are actually glued together, there are various "naked edges" down there, a naked edge is one that only belongs to one surface rather than being joined between 2 surfaces.

That's also why your bottom edge is fragmented into some different segments, my guess is that the network surface undulates slightly and kind of dips up here and there from the bottom planar surface.

You can see naked edges by setting up a shortcut key with a script on it as described here:
http://moi3d.com/forum/index.php?webtag=MOI&msg=6051.2

I'll take a closer look at your model to see why that has happened, but my guess is that some of the curves you are using to network with don't quite have their endpoints exactly on the bottom plane.

- Michael