The Australian Navy was called in to investigate a report from a snorkeler who confused a pair of sea urchins for unexploded anti-shipping mines. He was snorkeling at Mosman near Sydney when he encountered the creatures which he believed to be WWII sea mines.
The police, who had doubts, showed the man pictures of both mines and urchins, but the man insisted they were indeed UXO.
A maritime exclusion zone had to be established around the area until Naval experts determined that they were actually two large sea urchins among the rocks on the seabed.
Perhaps the snorkeler mistook the urchins' tentacles for the "chemical horns" of a sea mine. These horns are the firing devices which project from the spherical casing of a sea mine. These horns are made of a soft lead which covers a electrolyte-filled glass vile. When a ship bumped against one of these horns, the glass vile would break open, allowing the electrolyte to flow between two contacts, closing the firing circuit and detonating the mine.