Hickory trees, people.
Hickory nuts are encased in bright green cases that pop open into 4 sections and drop the nut out when the nuts are ripe. Sometimes the whole thing falls off the tree. Inside the case, they're a bit bigger than a golf ball. The sections of the case are hard and brittle and hurt like a you-know-what to step on, since the ends are pointy.
The shells of the nuts themselves look like little boats when they're split open. They also attract every crow and squirrel in the area. I don't know why, but crows seem to love hickory nuts. The squirrels and crows leave the little hickory shells all over the yard. Also, Hickory trees grow quite tall, though their trunk diameter doesn't get as impressive as, say, an oak tree does.
We have a very small hickory tree growing in our back yard right now. This is the first year it has made nuts. DH took one look at it and said "it has to go" because those nuts turn into projectiles if hit by the mower, and neither of us want to go outside with a bucket to pick every single nut that falls.
Barring hickory trees, sweet gums. Hundreds of prickly little seed balls that fall off come fall..yep. they're also a messy, messy tree.