A pair of divers on Indonesia's Komodo Island had a once-in-a-lifetime encounter this week when they crossed paths with a rarely seen megamouth shark. The sighting comes just two months after a six-metre megamouth was seen in Japanese waters. That specimen sadly died after it was penned for study, but to the best of our knowledge, this animal swam off in seemingly good condition. 

The footage was captured by Penny Bielich, who encountered the shark near Komodo National Park's "Shotgun" dive site. According to fellow diver Heikki Innanen, he and Bielich were the only people in the water when the animal cruised by.  

"The [shark] seemed in good health, and a little bit interested to see us down there," recalls Innanen. "It adjusted its course to straight at us and then passed around three meters away. We were only in water about eight metres deep at the time."

Innanen initially mistook the gargantuan passerby for a whale shark, but as its unique body came into to view, the nature of the encounter became abundantly clear. 

"When I saw the silver skin, all the options for big silver sharks went through my head," he says. "'Great white? Tiger?' After seeing the head it was very obvious it was a megamouth, so I started yelling through my regulator to get Penny to take video – nobody would believe us if we only had that grainy first shot!"

Revered as one of the region's most exhilarating dive sites, "Shotgun" lies nestled between Gili Lawa Laut and Gili Lawa Darat, two small islands located just off the northeastern tip of Komodo.

Strong currents combined with the local topography bring cold, nutrient-rich water up through the site, while pushing warm surface water down towards the deep ocean. This cyclical up-and-down welling gives Shotgun its second nickname: "the washing machine".

All that circulation makes this area the perfect habitat for marine life, from corals and other invertebrates, to schooling rays and sharks.  

Despite their impressive size, megamouth sharks (Megachasma pelagios) feed on some of the ocean's smallest inhabitants. Using 50 rows of tiny teeth, the filter-feeders sieve krill, jellies and other plankton as they swim. Many of these prey species regularly move from deep to shallow water, and local currents give them a boost towards Shotgun. It's possible, then, that the megamouth had simply followed its dinner to arrive at the dive site. However, deciphering the ins and outs of behaviour can be tricky when dealing with such a mysterious animal: the megamouth shark eluded discovery until 1976!

These sharks spend ample time at extreme depth – up to 1,500 metres below – and when it comes to working out the details of their ecology, we're only just scratching the surface. The species' daily movements are known from just one individual – and that shark travelled up from 180 to 20 metres overnight. It's not a lot to go on, but scientists believe all megamouths make these vertical trips to the shallows at night in search of food. Innanen and Bielich encountered the shark at 4:50 p.m., so this may have been the beginning of a foraging mission. That said, so little is known about these animals that it's impossible to say with certainty what is "typical" behavior. 

"The biology of the megamouth is almost unknown," says Dr Kazuhiro Nakaya, a professor emeritus at Hokkaido University who has done extensive work on megamouth sharks. "I can't say this is normal or not. However, it swims strongly and beautifully, and I feel this is their normal behavior. So beautiful and impressive."

Nakaya notes that while the tracked megamouth shark ascended in darkness, another specimen recorded from from Indonesian waters was attacked by whales during the day.  

It's possible the recently-spotted shark was sick or injured, but Dr Dave Ebert, who also studies these ellusive creatures, agrees that it appears healthy. "It looks like it is swimming just fine to me," he says. "An amazing sighting!"

The clip is among the first natural swimming scenes ever captured of a megamouth at sea – the first without nets in the vicinity – making it an incredible piece of footage. And that rarity isn't lost on Innanen.

"As my friend told me, this was the most amazing one minute I will ever have in my life, surely there won't be another encounter like this," he says. "Sometimes diving is not fair, and we just happened to be the lucky ones there at the right time, Penny was doing her eighth dive. She's my lucky penny."

Find out more about these incredible animals here: