Reports of giant anacondas date back as far as the discovery of South America when sightings of anacondas upwards of 50 meters (150 feet) began to circulate amongst colonists and the topic has been a subject of debate ever since amongcryptozoologists and zoologists.

Anacondas normally grow to sizes of 6 metres (20 ft), and 250 kilograms (40 stone) in weight. Although some python species can grow longer, the anaconda, particularly the Green Anaconda, is the heaviest and largest in terms of diameter of all snakes, and is often considered the biggest extant snake in the world. It is not uncommon for a fully grown anaconda to attack and kill a jaguar or caiman. The lengthiest reputably-measured and confirmed anacondas are about 7.5 meters (25 feet) long. Lengths of 50-60 feet have been reported for this species but such extremes lack verification and too add lack of large prey to support a super-large snake. The two only real reliable claims that can be found describe measured anacondas ranging from 24-37 feet although these remain unverified.