I scream for a bit, as one does when one has a harpoon in one's leg. I cling to the buoy for a while, and find that I still have my Mace, and the buoyancy aids, thankfully. I use the sharp edge of my mace to sever the harpoon's rope, and using them, I manage to make it back to shore, snacking on the occasional shark on the way.
I have my leg fixed up, but I can still barely walk.
Hmm. I miss being steel, so with the use of some ancient rituals and human sacrifice I reform myself as a being of steel once more. Of course, due to the likelihood of encountering water, I make sure it is stainless steel. As such, I can heal my leg.
I do some work to track down an ancient sailor who encountered R'yleh during one of its rare surfacings and managed to leave with his sanity mostly intact. I convince him to tell me where it was, and go to the area where it was.
Then I dive. And sink and sink and sink and sink. Until I hit the bottom. I sneak in, find the sleeping Cthulhu and gently detach it from the snoozing sanity breaker, then carry it away as stealthily as you can carry a Hill.
I then locate a terrible creature from the deep, a creature of dark power and unspeakable malevolence, and hit it until it agrees to return the Hill and I to the surface.
Upon the surface, I carry the suddenly much heavier Hill back onto land, and set it up in Paris as a modern art exhibit.
I call the exhibit MY HILL, and I am praised for my genius. Someone stealing it would simply cement my status as an artistic genius; after all, why else would someone steal it?