Length: 40 - 60 feet
Appearance: Reports are of a common Anaconda, easily found in the amazon rain forest, but in sizes that are not accepted for the species. The common anaconda is accepted to reach lengths of up to 18 feet, far from the 40 - 60 feet claims which come out of the jungle every two to three years.
Threat: High. Snakes are known to be opportunistic carnivores, eating whatever they can swallow. The width of a human's shoulders generally impedes a large snake's ability to devour a person, though it is known through several reports that Anaconda's and even some large Boa's have attempted to eat people. A snake the size of the giant anaconda, would have no difficulty in catching, killing, and devouring a human adult whole
Sightings are mostly anecdotal, stories from native tribes and the rare glimpse of a serpent with nothing around to help judge its size accurately. The fact that these snakes are aquatic, and the murkiness of the Amazon river can make it very difficult to determine a snake's size. You may be able to see the head and ten feet clearly, but there is really no telling how much of the snake is still hidden in the water. It could be six feet, or it could be another twenty. That the issue is a matter of size of a known animal rather than a new species, coloration doesn't vary either between the common and the giant anaconda.
While the giant anaconda is limited to the Amazon rain forests of South America, there is a current problem in the Florida Everglades with Reticulated and Burmese Pythons. The perfect climate and the lack of any natural predators means a lot of these snakes are quickly growing to threatening sizes that, while they may not be able to devour an adult human, they could still easily kill one, and it would be very possible that smaller children could disappear, only to turn up inside a large snake.
While most scientists will dismiss the idea of snakes that big, there are ones that size and larger in the fossil record, and reports of sightings continue to this day. Two facts which support the possibility of such large serpents. Snakes can continue to grow throughout their lives, based on the amount of food available, so with a constant supply of food, as is provided by farming communities in the Amazon now. Also, there is a large amount of the Amazon which remains to be explored in the current age, between 60 to 80% depending on some sources. So it's very likely there are large animals left to be discovered.
So if you travel to the Everglades or down to South America, be aware if you go swimming in the rivers. These ambush predators aren't picky about what they grab, even if they don't find out they can't eat their prey until after it's dead.