A giant squid provided a rare treat for onlookers in Toyama Bay when one swam into the harbour.
The 3.7m (12ft) cephalopod was much smaller than the 13m they can grow to.
It spent several hours in the harbour on Christmas Eve and was filmed by local divers.
Professional underwater cameraman Takayoshi Kojima told the BBC he rushed to the harbour when a marina manager called and he helped guide the squid to the exit to the sea, where it finally disappeared.
Japanese researchers took pictures of the elusive creature hunting 900m down, enveloping its prey by coiling its tentacles into a ball.
The images show giant squid, known as Architeuthis, are more vigorous hunters than has been supposed.
The images, captured in the Pacific Ocean, appear in the journal Proceedings of the Royal Society B.
Documentary companies have invested millions of dollars trying to film adult giant squid in their natural environment. These efforts have met with little success – though one team has managed to capture a juvenile on film.
Japanese fishermen have taken snaps of an adult at the surface but, until now, no one had obtained images of the animal in its deep-sea hunting grounds.
In their efforts to photograph the huge cephalopod, Tsunemi Kubodera and Kyoichi Mori, have been using a camera and depth recorder attached to a long-line, which they lower into the sea from their research vessel.
Below the camera, they suspend a weighted jig – a set of ganged hooks to snag the squid – along with a single Japanese common squid as bait and an odour lure consisting of chopped-up shrimps.
At 0915 local time on 30 September 2004, they struck lucky. At a depth close to 1km in waters off Japan’s Ogasawara Islands, an 8m-long Architeuthis wrapped its long tentacles around the bait, snagging one of them on the jig.
Kubodera and Mori took more than 550 images of the giant squid as it made repeated attempts to detach itself.
The pictures show the squid spreading its arms, enveloping the long-line and swimming away in its efforts to struggle free.
Finally, four hours and 13 minutes after it was first snagged, the attached tentacle broke off, allowing the squid to escape. The researchers retrieved a 5.5m portion with the line.
“It was exciting to get a live Architeuthis tentacle. It was still functioning when we got it on the boat,” Dr Kubodera told BBC News.
The large suckers repeatedly gripped the boat deck – and Dr Kubodera’s fingers when he prodded the severed appendage.
“The grip wasn’t as strong as I expected; it felt sticky,” he explained.
But while other researchers have suggested that Architeuthis is a rather sluggish creature, the images show it is in fact an energetic predator.
Dr Steve O’Shea, of the Auckland University of Technology, New Zealand, told the BBC News website that he was extremely pleased for the researchers.
Kubodera, he said, had “ever-so-quietly been working away in the background on this for a number of years”.
And Dr O’Shea, a world renowned expert on giant squid, added: “From the point of view of the public, who believe this squid is the largest, the meanest, most aggressive squid that we have – it is hugely significant.”
The Auckland-based researcher said now that the squid had been caught on camera, researchers could focus on other, lesser known squid species and on conservation.
Bottom-trawling by fisheries is destroying squid egg masses on the seabed, Dr O’Shea claimed. Evidence for this comes from an efficient squid predator – the sperm whale.
“Five of the species of squid that were staple in the diet of the sperm whale are recognised in New Zealand as threatened solely as a consequence of the effects of deep-sea bottom-trawling.”
“[Sperm whales] are returning from the Antarctic on their historic migratory route to one of the richest regions on Earth in terms of squid diversity. But the larder is bare and the poor things are washing up on the beaches here starved.”
The giant squid is by no means the largest known. Several other species, including the colossal squid Mesonychoteuthis hamiltoni, are thought to grow larger.