I've seen the big green balls every fall since we've lived here, and even had a few hit my car when driving by the trees they came from, but I never knew that they were anything potentially valuable, taste wise. When I learned that these were black walnuts, I was inspired to get off my duff and start foraging for them.
Black walnuts have a much richer taste than the regular walnuts you find in the store. They are also very expensive comparatively, so if you happen to have trees you can get legal access to, it's totally worth the time and effort to gather and process them. Don't be a jerk and harvest from private property or from a forest preserve without clear permission first.
To get to the black walnut itself, you need to remove the green vegetal layer that covers the hard shelled nut inside. The juice from this outer layer will stain *everything it touches* a dark brown, to black. So, wear gloves. That don't leak. Or this will be you:
Fun fact: Ten days later and my nails are still dark brown, though the skin has faded a bit. This is after multiple soaks in hydrogen peroxide and lemon juice.
The best way to remove the green outer shell of black walnuts is by stepping on them, and rolling back and forth until the nut inside comes out.
Then, rinse them under a hard blast to remove as much of the rest of the goo as possible.
Let the black walnuts drain a bit, and then spread in an even layer on a large cookie sheet or other flat platter, and let them dry for a few weeks. It's best if they are out of direct sunlight, and in our house they can become cat toys so we have to plan accordingly.
Black walnuts are very hard to crack. You could look at it as a challenge along the lines of Feats of Strength for Festivus, or just use a heavy hammer on a hard block. I've even heard of people running over them with their vehicles, but haven't tried that out for myself yet, so your mileage may vary (LOL!)