I bet you've seen it but do you know what it is?
We found this specimen on Sandy Neck in Barnstable today. In fact, we saw a lot of them.
When I was a kid my mom told me it was whale poop ;-) It isn't, though. It is actually an organism--or more correctly, a colony of organisms called zooids, encased in a layer or layers of cellulose. It feels a bit rubbery and can come in various shapes, sizes and colors though the average seems to be about 4-6 inches wide.
I found this information on Sea Pork to be helpful and this blog on sea pork has lots of interesting photos.