The water catacombs, were built at the same time as the more well know Milos Catacombs, in the first century AD, by pre-Christian Romans.  Originally dug into the soft cliff-face, they have been exposed by erosion, over the 1900 years since they were dug.  Each hole is one grave and some of the caves have several.  The door in the back of the cave) above the paddle of the green kayak) leads to a chamber with 5 graves.