The second-largest living fish is a gentle giant with some peculiar habits and a knack for instigating cryptozoological debates. Brush up on your basking shark trivia with these 10 tantalizing tidbits.

1. THEY’RE BUS-SIZED FILTER-FEEDERS.

The two biggest fish in the sea consume surprisingly tiny animals. Basking sharks can grow to be 36 feet long and weigh four tons or more. Within the world of fish, this impressive size is exceeded only by that of the enigmatic whale shark. Those of you with a shark phobia will be relieved to learn that neither species is a big-game hunter; on the contrary, they eat plankton, fish eggs, and other minuscule organisms.

Like whale sharks, basking sharks use filtration to capture their food. The gills of both fish are lined with bristle-like, 3-inch structures called “gill rakers.” When it comes to capturing food, these rakers are critical. To feed, a hungry basking shark opens its colossal mouth—which is 3 feet wide in adult specimens—and swims around at a leisurely 2.5 to four mph. Any prey that might be floating in its path are corralled into the mouth, ensnared by the gill rakers, and then forced down the shark’s narrow throat.

By filtration system standards, the whole apparatus is a model of efficiency. In just 60 minutes, a basking shark can strain at least 1800 tons of water through its gills.

2. PEOPLE USED TO CALL THEM “SUNFISH.”

Over the centuries, English-speakers have given these leviathans plenty of different names, including elephant sharks, bone sharks, and hoe-mothers (“hoe” being descended from the Gaelic word for dogfish). In Scotland and Ireland, the preferred moniker used to be "sunfish." However, by the turn of the 18th century, this name had become problematic because many people had started using it to describe the ocean sunfish, an entirely different animal with a compressed body and really weird teeth.

Then, in 1769, Welsh naturalist Thomas Pennant argued that the filter-feeder should be renamed the basking shark. Why’d he pick that particular name? Like most scientists at the time, Pennant believed that these huge fish hang around near the surface of the ocean in order to absorb solar rays. More recent evidence suggests they really do this because, at higher latitudes, plankton congregates just under the surface. Soaking up sun probably isn’t their main objective. Maybe it’s time for another name change.

3. BASKING SHARKS CAN GO AIRBORNE.

Boaters need to give basking sharks a wide berth. The giant fish might not be man-eaters, but they can still be dangerous. Biologists aren’t sure why, but basking sharks occasionally launch themselves out of the water Free Willy–style and come crashing back down with tremendous force. Maybe they do this in order to impress the opposite sex. Or perhaps breaching the surface helps them get rid of lamprey eels and other parasites that latch onto their skin. Regardless, you’ll want to maintain a respectful distance between your vessel and the sharks. In 1937, one breaching individual accidentally capsized a small boat that was passing through the Firth of Clyde, off the coast of Scotland. Three people drowned.

4. THEIR OIL-RICH LIVERS HELP KEEP THEM AFLOAT.

To regulate their buoyancy, most fish depend on a swim bladder. By regulating the amount of gas inside this organ, a fish can change or maintain its current depth. However, sharks don’t possess swim bladders. Instead, many species have enlarged livers that are filled with helpful oils. These oils sport high concentrations of squalene, a low-density hydrocarbon. Because it’s lighter than seawater, it gives the sharks buoyancy. As a way to compensate for their immense body size, basking sharks have evolved huge, squalene-filled livers—without which they’d sink like rocks.

Historically, basking shark livers were a valuable resource. In Europe and elsewhere, oils extracted from them became a common type of lamp fuel during the 1700s and 1800s. Simultaneously, the squalene was used as a key ingredient in perfumes, industrial lubricants, and man-made silk. Even today, it’s used to temper high-grade steel. To feed the world’s thirst for basking-shark livers, fishermen once slaughtered the animals en masse. Between 1946 and 1986, harpoon fisheries in Ireland, Scotland, and Norway killed 77,204 of them. Fortunately, numerous protective measures have since come into effect, and it’s now illegal to hunt basking sharks in many places.

5. GREAT WHITES AND ORCAS MIGHT BE NATURAL PREDATORS.

Are any non-human animals brave enough to hunt full-grown basking sharks? Maybe. Killer whale pods and great white sharks have been seen feasting on dead specimens. But it's possible these predators were merely scavenging the remains of basking sharks. No confirmed instance of either species attacking or killing the filter-feeders has been documented.

6. BASKING SHARKS WERE ONCE THOUGHT TO HIBERNATE ON THE OCEAN FLOOR.

In 1953, a pair of biologists suggested that the reason basking sharks seemed to disappear from northern European and American waters every winter was that they were swimming down to the ocean floor and hibernating. The idea spread like wildfire. “At this moment,” reads a 1962 New Scientist article, “there are probably great schools of these enormous fish quietly resting at the bottom of the sea.”

Engaging as such imagery is, we can now confidently dismiss the hypothesis. A 2009 paper published in Current Biology finally answered the riddle of where these sharks go during the chillier months. Near the Massachusetts coastline, the authors fitted 25 basking sharks with satellite tags. The tags started transmitting data from some unexpected locations. “When a tag popped up in the Caribbean Sea, I was really blown away,” co-author Gregory Skomal told National Geographic. A few of his sharks headed even further south—one specimen migrated all the way to Brazil.

Skomal and his co-researchers also learned that the fish frequented some very deep waters as they traveled. For weeks or months at a time, many of the sharks remained somewhere between 650 and 3300 feet below the surface.

7. THEY REEK.

Basking sharks produce a mucus-based slime that covers their skin. Presumably, it’s an anti-parasite defense mechanism that keeps lampreys and other freeloaders at bay. This might explain one of the sludge’s more unusual properties: extraordinary corrosiveness. Nets that come into contact with a basking shark’s hide tend to rot away as the slime burns through their natural fibers. And, as many ichthyologists have noted, the ammonia-rich cocktail also makes basking sharks stink to high heaven. In fact, the smell is so powerful that some fishermen claim they can even smell a fully submerged basking shark from a considerable distance.

8. JUVENILES HAVE HOOK-SHAPED SNOUTS.

The reproductive habits of basking sharks aren’t well understood, and nobody has yet figured out how long they usually live. But at least one thing about their life cycle is clear: juveniles look markedly different from older specimens. Youngsters have fairly long snouts that curve downward. When the animals mature, however, their snouts become straighter and, proportionately, a lot smaller. Such conspicuous differences between the age groups once led scientists to believe that adolescent and adult basking sharks represented two different species.

9. DESPITE THEIR MANY SIMILARITIES, BASKING AND WHALE SHARKS ARE NOT CLOSE RELATIVES.

Our oceans are home to more than 400 shark species. Scientists have divided these up into eight major groups. Basking sharks are classified as lamniformes, along with great whites, shortfin makos, and sandtiger sharks. On the other hand, anatomical and genetic data reveals that whale sharks belong to the orectolobiformes order, as do nurse sharks. Therefore, the filtration systems of Earth’s two biggest fish must have evolved independently—a phenomenon known as convergent evolution.

10. BASKING SHARK CORPSES HAVE OFTEN BEEN MISTAKEN FOR DEAD SEA MONSTERS.

An alleged sea serpent carcass that washed onto a beach near Scituate, Massachusetts in 1970 was positively identified by several experts as the rotting corpse of a basking shark. Seven years later, the Zuiyō Maru, a Japanese fishing trawler, hauled up and photographed a strange-looking cadaver. At first glance, it looked like the creature had a long neck, a small head, and four flippers. This led some to believe that the newly dead animal was really a prehistoric plesiosaur reptile. However, according to tissue samples, it was almost certainly a shark—and most likely one of the “basking” variety.

Over the past 200 years, many other so-called “sea monster” bodies have turned out to be probable basking sharks. Why does the same story keep repeating itself? Well, when the filter-feeders die, their lower jaws tend to break away from the rest of the body at an early stage in the decomposition process. The tail and dorsal fins are also among the first things to fall off. Consequently, a dead basking shark might look an awful lot like a long-necked, small-headed plesiosaur—or maybe some kind of sea serpent—to the untrained eye.