Anacondas are four species of aquatic boa inhabiting the swamps and rivers of the dense forests of tropical South America. The Yellow Anaconda can be found as far south as northern Argentina.
There is some debate about the maximum size of anacondas, and there have been unverified claims of enormous snakes alleged to be as long as 30–45 m (98.4–147.6 ft). According to Lee Krystek, a 1944 petroleum expedition in Colombia claimed to have measured an 11.43 meters (38 ft) specimen, but this claim is not regarded as reliable; perhaps a more credible report came from scientist Vincent Roth, who claimed to have shot and killed a 10.3 meters (34 ft) specimen. Based on documented evidence, Anacondas can grow to about 23 feet long.
There are some reports from early explorers of the South American jungles seeing giant anacondas up to 18.2 meters (60 ft) long, and some of the native peoples have reported seeing anacondas up to 15.2 meters (50 ft) long, but these reports remain unverified.
Another claim of an extraordinarily large anaconda was made by adventurer Percy Fawcett. During his 1906 expedition, Fawcett wrote that he had shot an anaconda that measured some 18.9 meters (62 ft) from nose to tail. Once published, Fawcett’s account was widely ridiculed. Decades later, Belgian zoologist Bernard Heuvelmans came to Fawcett's defence, arguing that Fawcett's writing was generally honest and reliable.
Historian Mike Dash writes of claims of still larger anacondas, alleged to be as long as 30 meters (98 ft) to 45 meters (148 ft) — some of the sightings supported with photos (although those photos lack scale). Dash notes that if a 50–60 ft anaconda strains credulity, then a 150 ft long specimen is generally regarded as an outright impossibility.
It should be noted that the Wildlife Conservation Society has, since the early 20th century, offered a large cash reward (currently worth US$50,000) for live delivery of any snake of 30 feet or more in length. The prize has never been claimed. Also, in a study of 1,000 wild anacondas in Brazil, the largest captured was 17 feet (5.2 m) long.
Recently an anaconda snake measuring over six meters and weighing nearly 150 kilos (330.8lbs) was captured in the backyard of an abandoned house in Parana, Brazil.