excessive fumes form under the car?

under the car, near side, mid/ rear there is what looks like a box, with pipes coming from tis box, one of the pipes is an exhaust. not the main engine exhaust. what is this box? some sort of heat exchanger? there is a diesal leak from what appears to be one of these pipes, i am assuming it is this that is causing the smoke. what is the likey cost, requirement to reppair?