It's the driling to the the oil pump. A similar one can be seen at the rear of the crankcase. The rear drilling intersects the side one and delivers oil to the oil filter cannister.
Since these plugs see full pressure from the pump, they can be difficult to seal. I remove them and spray the threads and passages repeatedly with alcohol-based brake cleaner. When oil stops emerging-and this can take a full day-I apply Loctite 272 (red) to the plugs and screw them back in. Give the Loctite time to cure and/or apply some heat to speed the process. You will need to jack up the back of the motor, but this can be done without removing drive shaft bolts, etc.