Shown in stunning and rare footage, researchers in the Gulf of Mexico filmed a giant squid in the deep sea as part of an expedition to document elusive marine animals.

The video, which lasts less than 30 seconds and was taken about 100 miles southeast of New Orleans, shows what appears at first to be a single tentacle appear from the darkness.

Then, the tentacle stunningly expands and is revealed to be the lower half of the deep-sea creature. The squid flows about in the water before slowly pushing back and returning to the darkness.

"What were once monsters to be feared are now curious and magnificent creatures that delight," the team of researchers wrote in a mission log after viewing the video Wednesday.

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The researchers say it's the first time a giant squid has been spotted in U.S. waters, and Edie Widder, one of the leaders of the group, told the Washington Post it was "one of the more amazing days at sea I’ve ever had."

The research team was funded by the National Oceanic and Atmospheric Administration's Office of Ocean Exploration and Research and set out to observe marine creatures in the so-called "midnight zone," some 3,280 feet below the ocean surface.

Using special cameras, the team sought to better understand how the lack of light at such great depths affected animals and their vision.

The Medusa, a camera that employs red lights invisible to most creatures at those depths, was used to lure the animals with its attached "e-jelly" display, which mimics the bioluminescence of a deep-sea jelly fish.

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Researchers were reviewing Medusa's footage last week when they caught the first glimpse of the squid's long tentacles.

"People quickly gathered around. We knew immediately that it was a squid. It was also big, but because it was coming straight at the camera, it was impossible to tell exactly how big," the team wrote in their mission log.

The animal turned out to be 10 to 12 feet long, and the scientists soon made the preliminary identification of the juvenile giant squid.

Despite heavy storms and a lightning strike to their boat, the team was able to send the video to Michael Vecchione, a zoologist at the NOAA National Marine Fisheries Services, who confirmed the animal's identity.

"We all proceeded to go slightly nuts," Widder told CNN. "We know so little about how these animals survive in the depths ... this helps us learn something more about how they hunt and their energy budget, but we need to know a lot more."

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A giant squid wasn't filmed in the deep sea until 2012 in Japan. According to Smithsonian, the largest ever giant squid recorded by scientists was almost 43 feet.

"Most importantly, we did not find a monster," the team wrote. "The giant squid is large and certainly unusual from our human perspective, but if the video shows anything of the animal's character, it shows an animal surprised by its mistake, backing off after striking at something that at first must have seemed appealing but was obviously not food."

Follow USA TODAY's Ryan Miller on Twitter: @RyanW_Miller