You just never know what you'll discover walking along the beach at night. For Tiphareth Aquarian, a stroll along Lanikai Beach in Hawaii on the evening of May 10 presented an encounter with an unusual creature. As many of us might do, she quickly took out her phone and captured a video.

"I saw this on Lanikai beach Wednesday night on the full moon," she wrote on Facebook. "I believe it's an Argonaut or Paper Nautilus? Has anyone seen one of these before? It had beached its self but I got it back in the water before it died."

As Aquarian later discovered, the strange pulsating creature was in fact an argonaut. Also known as a paper nautilus, this open-ocean species of octopus is rarely seen by humans.

"I don't think that they come in close to shore very often, or they're out at times when people aren't out in the water," Megan Porter, an assistant professor of biology at the University of Hawaii at Manoa, told Khon2. "So to have one wash up on shore where people can actually see it, it's actually rare."

As for the unusual shell above the argonaut, it's actually an evolutionary innovation unique to the species. Created by females, the chambered shell traps gas to maintain buoyancy and is used as a brood chamber to protect developing young.

You can see an argonaut in the open water as captured by divers below.

Michael d'Estries ( @michaeldestries ) covers science, technology, art, and the beautiful, unusual corners of our incredible world.