I've always called them "freeze plugs" and the ones I have seen the "dome" actually goes to the inside. When you can get to them you just pop one edge of it with a punch or something to flip it sideways and then hook a screwdriver in it and pop it out.
Like red58 says, Replace them all and don't knock it inside the block and leave it. I have seen this done on hard to reach plugs and its a no no. I replace one long ago and stuck my finger down inside the block and pulled out 2 more that were rusted so you can imagine what they do the water cirulation! Some will be rusted out right in the middle and some leak on the edge. Clean the rim where the plug goes and use just a "tat" of sealer and your good to go.
