There are some true mammoths out there, prowling the depths of our deep blue oceans. Some are big friendly giants, others are out for blood. What are the biggest fish in the ocean, and should we be afraid?

The size of a school bus and heavier than three elephants, the biggest fish in the world is so huge that it strikes fear into the heart of any living creature.

Keeping in mind that whales must be disqualified (they are mammals, after all), the giant fish we're taking a look at today is truly breathtaking.

So, who takes the animal record for world's biggest fish? What is the biggest freshwater fish, and which beastie scours the ocean with a toothy smile?

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Let's take a look at the biggest fish in the world.

The biggest fish in the ocean is, by far, the whale shark. A whale only by name, these giant sharks are known to live to extraordinary ages and enormous sizes. Becoming sexually mature at between 10 and 30 years of age, whale sharks are unique and fascinating creatures that have skin so thick (around 6 inches) they'd hardly feel a football at full-pelt.

Due to their immense size, adult whale sharks have no natural enemies, meaning that humans are the sole reason for their classification as critically endangered. They eat small sea-creatures like fish, plankton, and prawns, using their mouth as a giant sucker that pulls in their feed and filters out everything that's unwanted.

As a result, these giant fish pass around 1,600 gallons of water through their gills every hour, the equivalent of 33 bathtubs. While they are, indeed, sharks, they are some of the friendliest and least dangerous in the world. They may have massive maws, but they ain't going to try to eat you.

The longest and biggest whale shark ever to be found was a whopping 60 feet in length, with the species averaging around 30–40 feet. They are often more than 40,000 pounds in weight. Considering that their mouth alone stretches multiple-feet wide, it isn't such a surprise that they are the biggest fish in the world.

Interestingly, while the whale shark is the biggest fish in the world, it ain't the biggest animal to swim through our oceans. That reward would clearly go to the blue whale, which, at more than 100 feet in length, is also the biggest animal in the world. Luckily for the whale shark, though, the blue whale is a mammal, not a fish.

The beluga sturgeon, otherwise known as the great sturgeon, is the biggest freshwater fish in the world. Listed as critically endangered by the IUCN, these giant creatures are primarily found in the Caspian and Black Sea. They used to also be found in the Adriatic Sea, but are have become extinct in the region over recent years.

These giant sturgeons regularly reach more than 15 feet in length and 2000 pounds. The largest recorded beluga sturgeon was a female discovered in the Volga estuary in 1827, which recorded more than 23 feet in length and more than 3400 pounds in weight.

As the biggest freshwater fish in the world, the beluga sturgeon has become famous as victims of overfishing and poaching. If urgent action isn't taken, these giant beasties will soon no longer swim in European waterways.

Living in tropical and temperate oceans around the world, the ocean sunfish is one of the strangest and most interesting animals out there. Very few animals prey on these giant things, though sharks and orcas will try and have a nibble from time to time. They are predators themselves, though, and often eat jellyfish and small fish.

As the biggest non-shark fish in the world, ocean sunfish regularly reach as large as 10 feet in height and as heavy as 2000 pounds. They are absolutely massive, and incredibly odd-looking, resembling a sail or fan when viewed from the side. Part of the reason for this resemblance is the fact that they are weirdly flat and strangely shaped.

Whale sharks migrate hundreds of miles, only moving at a few miles an hour, often alone and deep in the blue abyss of our oceans. As solitary animals, they have come under threat from hunters and fisheries, becoming the victims of bycatch and killed en masse for their meat, fins, and oil.

The sad reality of this situation is that despite the efforts of various governments and NGOs, the largest fish in the world are slowly dying out. Only time will tell whether these giant creatures will be around in a few decades time.