2002 s80 oil leak

[toots1]

Hi
I discovered a massive oil leak today and it seems to be coming from the engine block dead center under the exhaust manifold,it looks like a 25mm hole not threaded, like a core plug or where a sensor should be.the oil leak isnt to bad until you rev the engine then it comes out under a fair bit of pressure.can anybody tell me what this hole is or if anyone has a engine diagram.the engine is a 2.4 petrol 20v.
thanks
Tony