The Númenórean Ruin is a structure found on Middle-Earth islands. They are the remnants of the great Númenórean empire of the Second Age. They are also found in the sea surrounding Meneltarma.
There are common small structures, and less common large variants, however the two are indifferent besides in size. These have no treasure in them whatsoever so they are useless (unless you need cobblestone or stone bricks, but that would be a lot of work for little payoff.)