Sounds like a great sci-fi flick. But it's not.

It's real and a nightmare for Japanese fishermen.

The massive sea creatures, called Nomura's jellyfish, can grow 6 feet (1.83 meters) in diameter and weigh more than 450 pounds (204 kilos). Scientists think they originate in the Yellow Sea and in Chinese waters. For the third year since 2005, ocean currents are transporting them into the Sea of Japan.

Monty Williams, a marine biologist at Alabama's Dauphin Island Sea Lab, said the jellyfish grow to an enormous size as they are transported by ocean currents. He said they stay together in packs and as they drift northward, they get caught in fishermen's nets.

"The sheer size of them, individually, makes them fairly spectacular," Williams said.