It's something you have to see to believe. A beach on the island of St. John in the U.S. Virgin Islands is overtaken by a millions of hermit crabs during the crustacean's mass migration, turning the sandy shore into a sea of clattering shells and legs.

Veteran videographer Steve Simonsen, who captured the phenomenon last week in a video that is going viral online, said he was covered with crabs in the shooting process.

"Crabs were crawling over my feet, arms, cameras. They crawled right into the lens hood," Simonsen said. "I was freaked out."

Simonsen, who specializes in underwater and nature films, was summoned by a friend, author Pam Gaffin, to the beach at Nanny Point to document the spectacular sight. Gaffin had been there since 6 a.m. after hearing the crabs rustling in the underbrush the night before in a mass exodus to the beach.

"[Gaffin] told me … there were millions and millions of them …she likened it to the migrations of Serengeti. I didn't need to hear anymore, I loaded my car with cameras and was out the door," Simonsen wrote on his Vimeo page, where the video was first posted.

Simonsen arrived at 10 am and joined Gaffin, who was snorkeling in the water.

"You couldn't walk …you had to pick out high rocks that no crabs were on. Or if a crab or four or five where on it, I had to brush the crabs off with my foot. I had to pick my way through," he said. "What I found out that the crabs were not interested at all in pinching. They were on a mission."

Hermit crabs, also known as soldier crabs, are found throughout the Caribbean islands and take part in a great migration en masse annually in August to mate. The crustaceans travel to the beach, leave their shells and enter the water to lay eggs, according to Smithosianmag.com. After spending two minutes in the water, Simonsen said the crabs turn around, return to land and make their way home.

"People who live on St. John know this happens in August. I've never been able to see it or know when it happens," said Simonsen, who plans to study the creature's migrations, moon phases, tides and stake out beaches next August to see the phenomenon again.

Simonsen is working on a project filming leatherback sea turtles that nest on St. Croix.