The picture and the title say it all . If you want to find this, go to SMP9, and go to the large, empty residence next to spawn. Look for the trees around that res, and you'll find it.
This reminds me of when Dragon Egg owners got a bedrock to place the egg on. Why didn't any of them ask for sponge?
Its cause the egg sits on bedrock when the dragon is killed. So it was decided that bedrock was ok for the owners to have.
Unlike aikar however, they don't have an incentive. Sponge gets placed as a joke, breaks "accidentally", and thus ruins the economy with such a rare item. Or something...