Removing old garage floor drain cover

[joatmon2]

In the middle of our 1949-era cement garage floor there is a round (approx. 6 inch) iron drain cover with small holes in it and the legend "Slop Sink" on it. The drain isn't working and I would like to remove the cover to clear it out. I have tried using a masonry chisel around the edge of the cover, but even removing a 1/4" of concrete around it won't budge it. The cover is quite substantial--even using a sledge hammer on the convex-shaped cover won't dent or crack it. Can anyone advise me about how to remove this cover? I don't know the shape of the unit under it--do I have to remove more of the concrete around it? Other thoughts? (I would rather not pour corrosive or other drain products down the drain, for the drainpipe goes onto the back lawn.)

Thank you.

[Pulpo]

I think removing more concrete is what you have to do unless you can cut it with a sawzall. Use the Ax blade if you try it.

[badeyeben]

If the drainpipe is visible outside could you clean it out from there? Use the sewer tape from the outside toward the garage?