HAT KHRAI, Thailand - The monster fish announced itself with four huge whacks of its tail, thrashing against the net that had trapped it in the pale brown water of the Mekong River.

It was a rare giant catfish the size of a grizzly bear, and it took five boatmen an hour to pull it in and 10 men to lift it when they reached the shore in this remote village in northern Thailand.

Only after their catch had been chopped into pieces and sold did they learn how special it was. At nine feet in length and weighing 646 pounds, it may be the biggest freshwater fish ever recorded.

But in one of the world's more surprising mysteries, nobody really knows which is the biggest species of fish lurking under the waters of the Mekong or the Amazon or the Yangtze or the Congo or the Colorado or Lake Baikal.