See anything interesting under the water this past weekend?

[ghosch]

Last weekend I did a couple of dives in Calhoun. Not a whole lot to see, but we found these strange outcroppings on the bottom in about 25' of water. At first it looked like a huge, old tree trunk partially buried in muck. On closer inspection, it obviously wasn't wood, as it had sort of a spongy consistency with lots of holes, like swiss cheese, and was dingy white/gray in color. It was a good 10-15' long and several feet high. We found a couple of them.

Anyone seen this stuff before and have any idea what it is?