(CNN) -- The first ever video footage of a giant squid swimming in the ocean depths is "an enormous breakthrough," according to a prominent marine conservationist who wrote a book about the quest to find the mysterious creatures.
"People have been searching for them for hundreds of years, literally," said Richard Ellis, the author of "The Search for the Giant Squid: The Biology and Mythology of the World's Most Elusive Sea Creature."
Scientists and television broadcasters released images this week of the 10-foot-long giant squid they had filmed deep in the Pacific Ocean, far off the coast of the Japanese mainland, during hundreds of hours of underwater research.
The discovery is significant for both science and mythology, in which giant squids have long played a notable role, Ellis said in an interview on CNN.
"We're going to learn how this thing moves," he said. "How it swims, what it does with its arms when it swims."
He described the squid as having eight arms and "two very, very long tentacles which it uses to grasp its prey." Its limbs have suckers lined with sharp teeth.
Monsters reminiscent of giant squids have been featured in fables and imagery through the ages, like the Kraken in Norse legend and the Scylla in Greek mythology.
"For a long time, people didn't even think they existed," Ellis said.
The carcasses of dead giant squid that washed ashore eventually proved that the creatures were real, but finding live ones in the wild has proved extremely challenging.
The first still photographs of one of the huge creatures were captured in 2004, and footage was taken of one floating on the surface of the water in 2006. But researchers and cameramen had never before managed to catch a glimpse of them in the ocean abyss where they live.
The mission that finally tracked down the creature involved 55 dives in two special submersible vehicles that spent a total of more than 285 hours far beneath the waves. Some of the dives went deeper than 3,000 feet.
The team of scientists and filmmakers on the mission came from a variety of institutions, including the National Science Museum of Japan, the Discovery Channel and the Japanese broadcaster NHK.
They used equipment including "ultra-sensitive camera systems with light invisible to squid, bio luminescent lures and secret squid attractants," the Discovery Channel said.
NHK and the Discovery Channel say they plan to air their programs about the squid sighting this month.