missing plug from front of engine block

i seem to be missing a plug from the front of the engine block . i dont know if its a wading plug or something to do with the crankcase . its in behind the pulley at the front of the engine block and is a threaded hole that looks like it needs something in it . any ideas would be appreciated , ive tried to upload some pictures but my computer wont let me find them

it is the wading plug ,as the 200 tdi is cam belt driven timing case runs dry ,plug is left out so as any leaking oil runs out without effecting belt

Thanks james for your info it was just the answer I was hoping for.