One of the parents was an acorn, from your description the other parent must have been a large squash, probably a pumpkin. The photos look like an acorn, but not many of those get to soccer ball size.

The bottom is getting flat now. You could stand it up. I have never had acorns here. Hmm... But that is also what I thought it looked like. Keep you posted.

I almost puuled that one out because it was near the pumpkins, glad I didn't. Take another picture of bottom and little ones as they grow.