Drillagent, could the tank have =imploded= during draining? Clogged vent could cause that, too. As cold as it is, gas build-up from bacterial action is a tad unlikely. Not saying that it couldn't happen... At the same time, the tank could have just had a bad seam and split from the weight if it was near-full. Maybe a mount or two could have broken and allowed the tank to flex, putting pressure on the seam. I do agree that, since the tank will have to be removed anyway, a new tank is probably the best way to go. Even if you weld the old tank, you're still fixing an 11 or 12 year old tank.
Lyle