It might well be a socket that covering the plug if you can see a 1/2" sq hole then it's a half inch drive socket stuck there.
The other alternative is that someone has tried to drill out and use an "easy out" to remove the broken threaded part.
See if you can have a better look down the plug hole in the head and get a better idea.
If all else fails it remove the Cylinderhead and get a machine shop to sort it.
Good luck
Maurice
February 2010