The local authorities were incredulous about the snorkeler’s claim, going so far as to show him photos of sea urchins to see if he may have mistaken echinoderms for mines. But the snorkeler was adamant about spotting the sea mines, so the local authorities called in the Royal Australian Navy to the scene.
As the authorities suspected, these “mines” turned out to be (pencil?) urchins.
To add insult to injury, the Navy was not immediately available to assess and “defuse” the mines, so Syndey Police was forced to zone off the urchins, watching over them overnight.
Better safe than sorry, right?