As LongBox says, it's the freeze plug. If you can block up the engine with a 2x4 under a sturdy part of maybe the front edge of pan, you can remove mount and poke hole gently in the freeze plug and twist and shout it out. Carefully tap a new one in there with a big socket that just fits inside it, till it's flush with block. You can coat the edge of it with gasket sealer if you wish, before hand. Maybe you know how to put in a freeze plug already? Didn't know.
