Imagine a great cavernous mouth, edged by hundreds of razor-sharp teeth. Powerful muscles work to rip flesh, crush bone and close the maw with terrific speed.
Such terrifying jaws are familiar from monster movies, fromJaws to Godzilla. But back in the real world, which animal has the worst bite of them all?
It is not just a matter of having a big mouth. The bowhead whale has the largest mouth of any animal, but it does not use it to bite. To really deliver a powerful bite, an animal needs a big mouth, lots of strong teeth, and powerful muscles.
To find the worst bite, we have to search on land, underwater and back in the mists of time.
The first thing to understand is that not every animal with teeth can bite. For instance, the common garden snail has thousands of teeth on its tongue-like radula. But it uses them to scrape, not gnash, so it cannot be said to bite.
The largest living [animal] using its teeth for catching its prey and tearing pieces off it is the killer whale
Similarly, the blue whale may be the biggest animal alive, but it does not have the biggest bite. There is a simple reason: it has no teeth. Instead it has plates of baleen, a net-like structure with which it filters food from enormous gulps of sea water.
The sperm whale looks more promising, as it is the largest toothed predator on Earth. But it is another red herring. It only has teeth on its lower jaw and tends to swallow its squid diet whole.
Oddities like these mean it is no simple task to find the biggest bite on Earth. According to Olivier Lambert of the Royal Belgian Institute of Natural Sciences, the best contender is not a whale – but it does prey on whales.
"The largest living [animal] using its teeth for catching its prey and tearing pieces off it is the killer whale," he says.
A lot of tests of bite force have been conducted from the safety of a desk
Despite their name, killer whales are actually dolphins. They are also known as orcas and can grow to 31ft (9.5m) long. They have around 50 conical teeth, which they use to rip apart prey – from seals to grey whale calves.
However, as any fan of nature documentaries can tell you, orcas usually hunt co-operatively to take down big prey. That means it could be a case of many mouths make light work, and their individual bites might not be especially powerful.
So maybe we want a lone predator, in which case we should take a look at the world's biggest predatory fish, the great white shark. Famous for their 300 blade-like teeth, which are continually replaced, these sharks certainly have one of the most feared bites on Earth.
But with a jaw made of flexible cartilage rather than solid bone, is it the most powerful bite? To find out, someone had to measure the force it exerts.
Measuring the bite of a predator like a great white shark might sound like a guaranteed trip to Davy Jones's Locker. Evidently scientists agree, because a lot of tests of bite force have been conducted from the safety of a desk.
In 2008, Stephen Wroe of the University of New England in Australia and his colleagues used computer simulations to estimate how powerful a great white shark's bite could be.
The most powerful bite recorded from a living animal belongs to the saltwater crocodile
For the largest sharks, the maximum bite force prediction was 18,216 Newtons. For comparison, our best bite with our second molars is estimated at a maximum of 1,317N.
Previous techniques for simulating bite force had considered the jaw as a two-dimensional lever, but Wroe used a method that went much further.
"3D finite element analysis has revolutionised predictions and analyses of bite force," says Wroe. "Just because an animal could hypothetically generate a particular force doesn't mean that it did. FEA allows us to predict stress and strain throughout the skull and jaws, as well as reaction forces, and thus allows us to predict overall mechanical behaviour."
Still, nothing beats measuring the bite of an animal in the flesh. A few hardy researchers have conducted tests on real live animals.
The most powerful bite recorded from a living animal belongs to the saltwater crocodile, according to a 2012 study byGregory Erickson of Florida State University in Tallahassee and colleagues.
It is not the teeth or long jaws of crocodiles that give them a big bite, but the ferocious snap
The team compared 23 crocodilian species, by persuading the reptiles to bite a metal sandwich on a pole. This "bite force transducer" measured how much pressure was applied across a pair of plates by the upper and lower jaw.
Erickson's team caught and restrained the animals, then placed the transducer between their back teeth where the bite forces were greatest. This meant only the "jaw adductor" muscle forces were measured, without any twisting forces.
The largest saltwater crocodiles delivered a crushing 16,414N, more than 3.5 times that of the previous record-holder, thespotted hyena. The crocodile's bite was slightly weaker than that of the great white shark – but the shark's bite was only simulated.
It is not the teeth or long jaws of crocodiles that give them a big bite, but the ferocious snap, according to Laura Porro of the Royal Veterinary College in London, UK.
There could be crocodiles out there with even stronger bites
"We think their high bite force is largely due to enormous jaw-closing muscles, particularly a muscle called the pterygoideus," says Porro. "If you look at a photograph of a large croc, these are the big fleshy jowls hanging near the back corner of their mouths."
This helps explain why the biggest crocodiles have such strong bites.
"Their jaw-closing muscles show positive allometry," says Porro. "As the animal grows, these muscles grow relatively faster than expected. Adult crocodilians bite harder than juveniles [partly] because they are absolutely bigger in size, but also because their muscles are relatively larger."
The crocs measured by Erickson's team were not the biggest specimens known, so there could be crocodiles out there with even stronger bites.
At the other end of the scale, the diminutive South American fish known as piranhas have a reputation for biting sizeable chunks out of their prey.
The shark bite researchers estimated the megashark's bite to be an "extraordinary" 108,514-182,201N
However, in a 2012 study, researchers measured the bite force of the black piranha to be 320N. That is feeble compared to the great white shark, even when you factor in the size difference.
That said, piranhas are not what they were. Around 9 million years ago, South America was home to giant piranhas called Megapiranha paranensisthat were up to 3ft (1m) long. The piranha researchers estimated that they had a bite force of 1,240-4,749N, and teeth that could crush bones.
This finding underscores the general pattern that bigger animals have the most powerful bites. For this reason, the giant prehistoric ancestors of our crocs, sharks and whales are estimated to have had the biggest bites of all time.
The whopping shark known as Carcharodon megalodon went extinct 2.6 million years ago. It may have grown to almost 66ft (20m) long, nearly 3.5 times the length of the biggest great white sharks.
Around 9 million years ago, South America was home to giant piranhas
The shark bite researchers estimated the megashark's bite to be an "extraordinary" 108,514-182,201N. That is enough to crush a small car.
For now, that is the strongest bite that has been formally estimated. But it may not be the strongest of all.
At the same time that C. megalodon prowled the seas, there was also a sperm whale called Livyatan melvillei. Named for the author of the ultimate whale tale, this huge whale lived in some of the same places as the mega-shark, and hunted the same prey.
It was a whopper: its head alone was 9ft 10in (3m) long. Not only that, unlike modern sperm whales it had functional teeth in both jaws. Those teeth may be the largest of any animal: they were 14in (36cm) each.
Very powerful bites can be reasonably expected for this animal
"With such skull and teeth sizes,Livyatan melvillei is undoubtedly one of the biggest bites of all time, if not the biggest," says Olivier Lambert, who was part of the team that first described the giant sperm whale.
So far, nobody has applied the finite element analysis technique used to model C. megalodon's bite force to L. melvillei.As a result, we cannot compare their estimated bite forces.
However, "it would definitely [be] worth trying," says Lambert. "Considering the jaw, teeth and skull size and proportions, very powerful bites can be reasonably expected for this animal."
So if you travel far enough back in time, there may be some fact to the fiction of Moby Dick.
Join over five million BBC Earth fans by liking us on Facebook, or follow us on Twitter and Instagram.
If you liked this story, sign up for the weekly bbc.com features newsletter called "If You Only Read 6 Things This Week". A handpicked selection of stories from BBC Future, Earth, Culture, Capital, Travel and Autos, delivered to your inbox every Friday.