In 1975 the entry level diving course that we taught was of four weeks duration and included two evening theory sessions per week with the Saturdays and Sundays given over to practical diving instruction. A standard part of the course included boat diving from a former commercial dive vessel, the ‘Salvus’. (I was reminded of this during a recent ferry trip that took us from Manly across Sydney Harbour and close to uninhabited Shark Island, named after the creature whose shape it is said to resemble.)
Towards the end of each course, we would often pull into the short jetty on Shark Island and allow the students, swimming in buddy pairs, to explore the waters around the island while we – my assistant and I – monitored their bubble trails from the deck and cabin roof of the ‘Salvus’.
My Assistant Instructor was Bob Cason – a man who would later head up IANTD in Australia and become one of the country’s foremost tek diving pioneers but who, in 1975, was still a young Naval Lieutenant.
Ten minutes or so after the first pair had entered the water they were back on the surface indicating that we should motor over and pick them up. The metal ladder was dropped over the side and as they scrambled back into the boat they announced that they had found a mine and having surfaced immediately above it had – thanks to the course navigation component – taken a quick visual triangulation fix on its position. Their excited description was of a large round object with “horns” identical to the mines that they had seen the previous week on a courtesy visit to the RAN’s Diving Museum at HMS Penguin, in Middle Harbour.
(I should point out that during WWII, Japanese midget submarines had initiated an attack on shipping in Sydney Harbour and that it was quite likely that the odd piece of unexploded ordnance was still resting on the sea floor.)
A succession of hefty bangs on the steel ladder with a hammer quickly brought the remaining divers back to the surface. Once they were all safely aboard Bob and I decided that we should confirm the sighting before reporting it to the authorities.
As the Instructor and nominal skipper of the boat, my opinion carried more weight than Bob’s. And as I was quick to point out to him, my role demanded that I remain on the surface and direct operations. Bob was kitted up and dropped over the side while I manoeuvred ‘Salvus’ to what I estimated to be a safe distance.
He surfaced a little while later carefully clutching the “mine” in both hands. The students – white-faced – confirmed that it was, indeed, what they had seen. We agreed with them that, Yes, it was a mistake that anyone could have made and that, Yes, the spines on Pencil Sea Urchins could, at a pinch (by overlooking their respective sizes) be mistaken for the ‘horns’ of a mine, before – privately – deciding that the course could, perhaps, be improved upon by the inclusion of a critter ID component into the programme.