It was eaten by Leeroy J. Connor.
They can drastically increase it making it extremely hard and heavy or decrease it to the point of utter intangibility. A problem seen with this power is that it can possibly erase matter by reducing it's density to zero which would cause it to disperse into the atmosphere. The user can also alter their own density to the same rules. Major weaknesses to Water, Haki, and Kairoseki are naturaly present but no other specific weaknesses are found at this time. The user can also crete tectonic shockwaves by altering their density to high enough levels and stomping the ground.