Nerida Wilson couldn’t take her eyes off the computer screen. Some 2,000 feet beneath the research boat she was aboard, a creature drifted past in the shape of a vast, galactic swirl. By her team’s estimates, it was 150 feet long.

“It looked like an incredible U.F.O.,” said Dr. Wilson, a senior research scientist at the Western Australian Museum.

She and her colleagues documented this organism with the help of SuBastian, a remotely piloted deep-sea robot, during a March expedition on the Falkor, a research vessel operated by the Schmidt Ocean Institute. Their mission was to understand what lives in the deep waters off Australia’s western edge. And the coiling stringy mass they had just found was a siphonophore, the first spotted off Western Australia and potentially the longest organism in the sea.

The longest previously known marine creature is the lion’s mane jellyfish — its tentacles can be up to 120 feet long. By comparison, blue whales, while the most massive creatures ever to have lived, are nearly 100 feet long.