put a chain around it, fasten the other end of the chain to the trailer hitch or bumper of a heavy-duty vehicle (pick-up truck, etc), & pull.

Here, it would cost maybe a 12-pack of beer...

lol. I dont have that luxury here in NYC.

I realize it's in the way of garden expansion. Necessity being the mother of invention - fill back in around it and use it as a plinth for a large decorative pot or urn for a floral focal point in your vegetable garden.

That hunk of concrete looks to be almost too much even for a sledge hammer.

Would it be a good idea to dig a deep whole and try to bury it deep down?

I was able to remove it with only a shovel.

Dug to the right of it in order to give it room to tilt onto its side. Once I did that it, I flipped and rolled it out of the soil. Took a little effort but worth having that extra space.