That stuff visible in the bottom of the hole is what's left after it was broached for an internal hex shape. That's all gone now (stripped) so you're looking at an extractor to grab and turn it. On the plus side it's likely not all that tight/stuck as it almost certainly is an o-ring boss type of seal. I mean, someone stripped it, but they may have stripped it impacting it in, not trying to get it out.
If you try an extractor and it doesn't want to break loose fairly easily, id get a chisel and smack a good little notch straight into the side of it, and then angle the chisel and start trying to drive it counter-clockwise. If you're nervous about damaging the sealing surface you could also take the extra step of cutting a slot on top from center hole to edge perhaps halfway through the material with a small cutoff wheel on a dremel before trying to start driving it CCW.
If you're a comfortable welder, even easier would be weld a decent sized nut onto it and just turn it with a socket.
It's a bit of a fishing expidition, though. 99% odds that it is a simple plug and all getting it out will do is let you replace it with one that isn't stripped. And practice your stripped fastener removal skills.