There are likely more than two million distinct species of animals alive on planet earth right now. This number includes some 15,000 types of mammals, nearly 10,000 birds, and many thousands of reptiles and amphibians, too. And of course it includes well over one million known insects, with the likelihood of at least a million more yet to be discovered. And what with evolution and extinction, the animal world is ever evolving, with new species popping up and older lines ending.

One place rich in biodiversity is of course the ocean. There are at least 20,000 extant species of fish (this includes freshwater fish, of course) at present. These fish share the waters with various other aquatic species, and together the many creatures of the seas (also known as… sea creatures) help to make the ocean a vibrant, fascinating place. And some of them help fuel your darkest nightmares. That’s so because for every lovable dolphin or cute little clown fish, there are also many animals swimming or scuttling about that are absolutely goddamned terrifying.

If you’re going to take a swim with any of the animals on this list, you might want to consider packing a spear gun and some thick gloves. And maybe a shark cage. Or maybe consider just parking yourself on the couch for a Netflix binge instead.

15. The Anglerfish

The chances of a human being and an anglerfish coming into conflict are pretty slim, really. Most members of this order are deep-sea denizens that live thousands of feet below the surface of the water, while most divers only go down a couple hundred feet at most. And as an anglerfish can grow to weigh more than 100 pounds and boasts a mouth filled with dagger-like fangs, that’s definitely for the best. The anglerfish uses a luminescent protrusion on its head to lure in prey (thus its name, “angler,” like a fisherman) and looks like something out of a terrible dream.

14. Giant Squid

Up until the 19th Century, the existence of the giant squid was still debated in many circles. These massive, mythical animals had been described by Ancient Greeks, allegedly spotted by Medieval mariners, and had long been a part of the lore of the seas. And until a number of actual carcasses were recovered in the 1800s, we didn’t know for sure if the giant squid was a real animal or simply a terrifying legend. Turns out it’s sort of both: it’s a terrifying real animal. These beasts can grow to measure nearly fifty feet long from the top of the head to the end of the tentacle, and weigh more than 600 pounds.

13. The Electric Eel

Here’s a little tidbit that might help you understand just how crazy the existence of the electric eel really is: the Electrophorus electricus is the only animal in its genus. That’s right, scientists had to come up with a brand new genus just to describe this thing. While technically a fish, not an eel at all, this long, tube-like animal looks and moves in an eel-like fashion. It can grow to more than six and a half feet in length, and as the name suggests, indeed this fish can actually generate an electric current. A mature electric eel can generate more than 860 volts for a brief moment; that’s enough to painfully shock a human and stun or even kill smaller prey.

12. The Barracuda

A barracuda does not see you as an enemy or as a meal. But that does not mean one of these large, ferocious fish might not take a chunk out of your arm or leg as a result of a misunderstanding. Barracuda, of which there are more than 25 species, many of which grow to more than five feet in length, are opportunistic hunters, attacking all sorts of live prey and gladly scavenging on dead stuff they find floating in the water. The trashing leg of a swimmer may well be mistaken for a tasty tuna, and their plethora of fang-shaped teeth will do the rest.

11. The Japanese Spider Crab

A normal crab is generally to be avoided. They have those claws, after all, and most species of crab are all to happy to pinch anything that comes within range. As it happens the Japanese Spider Crab is generally considered to have a rather mild temperament when compared to many other sea creatures, preferring to hide away than fight when possible. So why is this gentle creature on our list? Because it’s a gentle giant. As in a claw-to-claw leg span of as much as eighteen feet. Yeah, that’s right, this crab can grow bigger than your car.

10. Portuguese Man ‘O War

Sometimes an animal’s name belies its true nature. The Hellbender Salamander, for example, is absolutely harmless to humans. Then you have the Portuguese man ‘o war. These things are harmless to humans. In fact, the venom in the mighty tentacles of this unique jellyfish-like creature can cause agony or even death. What’s more, the trailing tentacles of this thing can reach a hundred feet in length. And really, I should say these things, to be precise: the man o’ war is made up of thousands of polyps that attach to one another and live co-dependently to approximate the function of a single animal. So it’s basically like the borg.

9. The Wolffish

Also known as the devil fish, the wolf eel, and the seawolf, the wolffish is an animal that you probably won’t ever have to fight, but to which you would probably lose the battle if that ever came to be. These fearsome creatures get their name due to their massive wolf-like teeth, which include multiple massive canine-shaped teeth in the front of the mouth and grinding molar-like teeth in the back. Like a wolf, get it? They don’t eat other fish, but instead grind up mollusks, clams, crabs, and other animals you would think would be protected by hard shells. The wolffish can also produce a natural antifreeze, which is itself nuts.

8. The Greenland Shark

Greenland sharks are, like most animals in their family, apex predators that live on a diet of other fish. They are one of the largest shark species alive, growing to lengths recorded at more than 21 feet, which is comparable to the largest great white sharks ever known. But the strangest, most unsettling thing about the Greenland shark is not its size or its rows of teeth, but simply its lifespan. These giant predators can live for hundreds of years; in fact, their life span may well be as long as 500 years. Think of it: a Greenland shark alive today might have been devouring hapless prey back when Luther was nailing his theses onto the church door. The longevity is attributed to frigid waters and a slow metabolism.

7. The Sea Snake

As if it’s not enough that venomous snakes are lurking in the desert, the jungle, and underneath the woodpile; they just had to set up shop at sea, too. Sea snakes are, in fact, some of the most venomous snakes on the planet. Many can kill much larger prey (including a human who does not get rapid treatment) with their bites. Sea snakes are unique in that most are fully adapted to life in the water, never venturing onto dry land, yet they are still air breathing and must surface regularly. Thus most of these creatures live in shallow waters, often among the nooks and crannies of coral reefs. So just where snorkelers like to go. Great.

6. The Barreleye

Here we have another animal that presents almost no threat to a human, yet which can surely prevent you from sleeping well at night once you have seen one. And indeed seeing is the name of the game here. Barreleye, also aptly known as spook fish, have eyes that can be directed forward or straight upward. When pointed up, the eyes can see through the transparent, dome-shaped head of the fish, watching for prey above. That’s right, this animal as a see through its own head. Fortunately, its mouth is actually rather small, and it mostly eats diminutive prey or even scavenges its food.

5. The Blue-Ringed Octopus

In general, the octopus is a rather unsettling creature, specific species notwithstanding. They have all those long arms (eight, for the record), that strange head, and there’s the whole ink thing, too. One would think, though, that the blue-ringed octopus would actually be one of the least frightening sea creatures of the genus due to it size. These critters only get to about eight inches in length, in fact. So why is the blue-ringed octopus scary? It is so venomous that its bite can kill a full grown adult. The blue-ringed octopus produces enough neurotoxin to kill about two-dozen people at any given time.

4. Frilled Shark

As you probably know, most sharks have lots of teeth. And often these teeth are distributed across several rows. In the case of the frilled shark, there are more than twenty rows of teeth in that goddamned mouth. And each tooth is like an inch-long needle. It’s not something you want biting you, basically. The shark gets its name from its many layers of frill-like gills, and might have been the inspiration for tales of mythical sea serpents due to the long body that contorts and twists with ease. Frilled sharks aren’t huge, usually measuring around six feet long, but they are terrifying to behold.

3. The Giant Oceanic Manta Ray

Picture this: you’re swimming along, enjoying a nice undersea adventure thanks to your scuba gear, maybe snapping some pictures of a reef or a sunken ship, when suddenly the sun above you is blotted out. You look up to see a creature measuring more than twenty-three feet across and weighing in at about 3,000 pounds. That indeed is how large the giant oceanic manta ray can grow. These massive rays don’t pose much risk to humans, and nothing much puts them at risk, either: giant oceanic manta rays are seldom on the menu for any predator. They also go where they want, with a habitat that wraps the entire globe save for colder polar waters.

2. The Gulper Eel

The gulper eel is also sometimes known as the pelican eel due to the elastic pouch like portion of its lower jaw. This flexible membrane put together with how wide it can open its mouth allows the gulper eel to ingest prey much larger than most animals of a similar size. Its flexible belly also allows it to digest larger prey, much like the case with most snakes. Gulper eels grow to measure more than six feet long and spend their days swallowing crustaceans and fish whole. Now imagine if they grew to a few dozen feet; yeah, you’d never swim again.

1. The Great White Shark

No surprise here, right? There’s a reason the great white shark is the first animal most people think of when they think about scary ocean animals: these things are scary. They grow to as long as twenty feet from snout to tail, they can weigh more than 4,000 pounds, and they are the cause of the highest percentage of recorded shark attacks on human beings. Also there’s the whole Jaws movie franchise thing. Great white sharks often live more than 70 years, during which time the average individual shark likely eats well over 110,000 pounds of meat.