What Am I?

Ok, so I picked up this weird looking seed from some tree. At first I thought it was an egg... it's the size of a large nectarine, and from its incredibly light weight, I assumed it was hollow. I took it home and decided to crack it open with a hammer. After a few solid pounds, the egg seed remained complete.

Eventually, I had to use a chisel along with the hammer to break it open to see that the inside was some really un-dense wood. I was wondering if anyone knows what this tree is that produces these super lightweight egg seeds. I didn't have my camera on me so I couldn't take a pic of the tree or its leaves. I'll have to do that next time.

Anyway, here are shots of the egg seeds... I know it looks like some sort of mushroom, but believe me, it's not a mushroom.