VIID is what it's called. It has vertical tubes for it, and you should put load the mines ONLY in those, otherwise they will indeed blow you up. For other boats, you should ignore them and never try to load them.
These are the biggest VIIs you can get, cause they have a separate 10m compartment added specially for the mine tubes.
http://en.wikipedia.org/wiki/German_...rine#Type_VIID
I can't remember exactly when that sub is available, but look around it on the flottilla lists. I believe you can only start a new career with it - you don't get offered one as a transfer in the regular careers. It's a special career for it that Lurker designed - haven't tried it yet, but seems like a pretty cool idea!