A large human-made monolith has been discovered underwater in the Pantellaria shoal, submerged since the end of the Ice Age.
E. Lodolo & Z. Ben-Avraham. A submerged monolith in the Sicilian Channel (central Mediterranean Sea): Evidence for Mesolithic human activity. Journal of Archaeological Science, 2015. Freely accessible → LINK [doi:10.1016/j.jasrep.2015.07.003]
Other source: Pileta de Prehistoria.
While it is hard to argue that the monolith is not man-made, as it has three holes of the same size in non-random positions, I would take with a pinch of salt the claim that the would-be standing stone or menhir has been there since the 10th millennium BCE, when the shoal was flooded by seawater.
Instead I would consider the following scenarios as plausible:
- The land could have been at higher absolute altitude in the past and sunk because of local techtonics. It is, we must not forget, a very active geological area.
- The monolith could have just sunk when being transported on a ship of some sort between islands. The ship, made of wood and ropes would leave no obvious trace.
So I'd rather imagine the stone to have been produced in the Chalcolithic Megalithic context that has some relevance in the area, very especially the fascinating case of Maltese Megalithism, which spans between 3600 and 700 BCE.