Bad Coolant Leak!!!

ok guys...i have a 90 300zx n/a. i woke up this morning to a small puddle of coolant sitting right in front of my left front tire. My car has never had a problem with anything leakin before so i was like GREAT, what now?!?!?!
anyways i check the fluid and it was fairly low so i add more to it and all the sudden antifreeze just starts gushing out like someone took a hose too it.
well...tonight i took the the underbody liner off and there is a fairly large black cylinder right next to my coolant resovoir. and coolant is just gushing down the side of it. i cant really see any hoses leading from one thing to another but on the same side of that cylinder there is a hose that runs straight up the side. i have researched this thing all night and kind find a pic of it or anything. i dont even know if this part has anything to do with the coolant lines because i couldnt really see any hoses attatched to it except at the very bottom and i cant tell where it leading too. do you guys have any idea what this black plastic cylinder is? ill try and get some pictures.

does it look like the overflow tank for the radiator?

If so it could have just had a line on the top pop off, or a crack formed in it. Or it could be coming from somewhere else and is finding its way down to that.

The only thing I can think of that it could be in the same area would be the carbon canister. But mines a TT and I can't really get a good look under it right now.

well i took another look at it today and did some hardcore investigating and it started to leak all over. from the left side of the car to the right side. the reservoir is completely full. so i filled up the radiator with fluid and as soon as pressure start to build the coolant started gush out all over. so i pulled the radiator and am takin it to a shop tomorrow. the radiator is less than 1 year old. well thats just my luck. get ready to do a brake upgrade and i got to get my radiator fixed. thanks for your replys guys.