They live hundreds of feet below the sea. A formidable predator that can rip its prey to pieces.

The giant Humboldt squid have returned to the waters of Southern California, and they're bigger and more plentiful than ever.

Fishermen are thankful, but biologists are worried.

"I have nearly a thousand dives with these animals and I have been either tested or full out attacked about 80 percent of the time," Scott Cassell said.

Cassell has been studying the Humboldt squid for the past 13 years.

"These animals are some of the most mysterious and unknown species in the world," Cassell said.

Cassell has even made a movie about his diving adventures, underwater excursions that require an armor plated suit.

"I have felt my life was in danger several times with the squid, but knowing that the cable and the armor I was pretty much impervious to the damage," Cassell said.

But Cassell, like other marine experts, says something is not right.

For the third time in ten years, massive amounts of Humboldt squid have been flourishing in the waters of Southern California.

"There is more population of Humboldt squid than is naturally propper," Cassell said.

In Newport Beach, fishermen climbed aboard the Freelance, eager for their chance to land a jumbo squid.

Captain Damon Davis knows exactly where the squid are running.

"What's unique about the last couple times they've been around is they've been huge, they've been big, they've been 20 up to almost 40 pounds, where in years past they've been five to 10 pounds," Davis said.

The fishermen use lures that glow in the dark. The squid can't resist.

Once the boat finds the school of squid, it only takes about 60 seconds before they start landing squid after squid.

The fishermen don't know why they're back, they're just glad they're back.

But Cassell says the declining number of sharks is probably what's behind the squid invasion.

"Their population has increased a little bit because we've wiped out their predators," Cassell said.

The fishermen aboard the Freelance caught more than 200 squid on Thursday.