If you're scared of snakes it might be wise to avoid Baie des Citrons, New Caledonia: the popular diving spot is alive with venomous greater sea snakes (Hydrophis major). Not everyone is bothered by their presence, however. We only know this thanks to a group of women in their 60s and 70s who have supplied scientists with photographs of their close encounters.

Dr Claire Goiran from the University of New Caledonia and Professor Rick Shine from Australia’s Macquarie University have been studying the turtle-headed sea snake at Baie des Citrons for 15 years. Among their discoveries is that dark patches in the skin absorb heavy metals, which the snake dumps when it sheds its skin, and that the snakes have been growing darker in response to rising pollution.

While studying their small, non-venomous and common subjects, Goiran and Shine also saw greater sea snakes, which can grow to 1.5 meters (5 feet), in the same waters; however, only about six times. Even when they sought H. major specifically, they only saw about 10 a year. The snorkeling women managed to find 250. 

Professional biologists can't be everywhere at once, so they are increasingly calling on the assistance of amateurs to keep track of the populations they are trying to study. Few, however, have had the help of as unusual a group as Goiran and Shine turned to. The seven women, who swim in the bay regularly calling themselves the “fantastic grandmothers” started taking cameras into the water and sent photographs of any greater sea snakes they encountered to the scientists.

Helpfully, greater sea snake markings are so distinctive individuals can be identified easily. Goiran and Shine compiled an album of 249 snakes, far more than anyone anticipated frequented the waters.

The seasonal nature of the snake's breeding pattern has been published in Ecosphere. Goiran claims this exceeds our knowledge of any of its close relatives.

“Remarkably,” Shine said in a statement, “they found a large number of lethally toxic sea snakes in a small bay that is occupied every day by hordes of local residents and cruise?ship passengers – yet no bites by the species have ever been recorded at Baie des Citrons, testifying to their benevolent disposition.”

While it is certainly inadvisable to approach an unknown venomous snake, Shine told IFLScience even the greater sea snake’s relatives maybe placid. “I hear lots of stories about “aggressive” sea snakes from divers, but I’ve never seen anything from sea snakes (of any species) except curiosity. In winter, when the males are searching for females, they will often approach us closely and tongue-flick us to make sure we aren’t female snakes … but as soon as they work that out, they swim away,” he said.

Shine told IFLScience the team has yet to investigate if the greater sea snakes are also getting dark in response to pollution, but he thinks their greater range may reduce exposure.