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Waterfalls are still a scientific enigma. Though people have admired their beauty and power for millennia, only in the last few decades have geologists focused on the forces that give rise to them and the unique ecosystems they support. Here’s the word on waterfalls, from the science behind them to their impact on art, culture, and industry.

Not All Water That Falls Is a Waterfall

You might think that all it takes to create a waterfall is hydrogen, oxygen, and gravity, but not every downward-flowing stream fits the bill. One guidebook suggests a waterfall is “a more or less vertical stream of water that flows over the edge of a cliff that has eroded away.” Water flowing over uneroded rocks, meanwhile, is considered a rapid. In addition, most experts agree that a fall has to be a minimum of 5 feet tall to qualify as a waterfall, and it needs to be fed by a stream, river, or creek that is replenished at least annually, such as by rain or melting snowpack.

Waterfalls Create — And Are Caused by — Erosion

Scientists used to believe that waterfalls were caused only by external geological or climate forces, but recent research has demonstrated that waterfalls can actually create themselves. Streams and rivers erode the rock over which they flow unevenly, and carve paths of least resistance over the landscape. Whether it’s washing across vertical or horizontal planes, water wears away soft rock like limestone and shale faster than hard rock like granite or basalt. Eventually, all that’s left is hard rock in ledges, steps, or walls, which creates the topography for a waterfall.

Waterfalls Can Be Categorized by Structure

The beauty of waterfalls lies in the way they flow over varying kinds of terrain, and many can be categorized according to their structure. Among the most scenic are plunge waterfalls, such as Bridalveil Fall in Yosemite National Park, which flows over a cliff without making contact with vertical rock. Tiered waterfalls flow over consecutive ledges of rock; Kaaterskill Falls’ two dramatic drops inspired painters of the Hudson River School in the early 19th century. A block waterfall, like Niagara Falls, occurs when a massive river flows over a cliff in a giant sheet. Horsetail falls make some contact with the vertical rock behind them and create picturesque sprays as they flow downward. Makahiku Falls in Maui, Hawaii, is one spectacular example. Other types include cataracts, sluices, fans, and waterslides.

The World’s Largest Waterfall Is Underwater

Earth’s biggest waterfall is under the surface of the Denmark Strait, the waterway separating Iceland and Greenland. How, you might be asking yourself, can a waterfall form completely underwater? The answer is in part a quirk of geomorphology — namely, a troughlike formation in an oceanic mountain range. Because cold water currents are denser than warm water currents, they sink downward, and in this particular spot cold water collects in the trough. Eventually, it overflows the rim of the trough and then sinks downward against the mountainside — and boom, underwater waterfall. The rim, called the Denmark Cataract, is 11,500 feet tall, making this waterfall at least three times as tall as its tallest counterpart on land.

There Are Some Pretty Big Waterfalls on Land, Too

Among land-based falls, Angel Falls in Venezuela takes the crown. Known as Kerepakupai Merú in the Indigenous Pemon language, the waterfall plunges 2,648 feet in a continuous flow from the Auyan Tepui sandstone plateau in Canaima National Park. (A 1949 National Geographic Society expedition calculated the falls’ height at 3,212 feet, but the measurement methods are disputed.) The falls are so tall that the huge volume of water surging from on high vaporizes before reaching the ground.

Most waterfall surveyors consider Angel Falls to be the tallest uninterrupted waterfall on Earth, but some argue that Tugela Falls, in South Africa’s Drakensberg Mountains, supersedes it, with a height that may be 3,225 feet over at least five segments. And then there’s Victoria Falls (also called Mosi-oa-Tunya), along the Zambezi River bordering Zimbabwe and Zambia, which is considered the world’s largest sheet of falling water. It maxes out at only 344 feet tall, but it’s more than a mile wide.

Waterfall Tourism Was a Thing in the 18th Century

Tourists flocking to famous falls didn’t start with the thousands who show up for “firefall” in Yosemite (when the setting sun in February appears to set Horsetail Fall on fire) or even with newlyweds at Niagara. European travelers of the Romantic era — roughly the late 18th century to the 19th century — also made a point of visiting especially scenic waterfalls, which they saw as paragons of the sublime. The waterfalls at Tivoli near Rome inspired tourists as well as moody, atmospheric paintings. The Marmore Falls near Terni in Italy — constructed by Roman engineers in 271 BCE, and at 541 feet the tallest human-made waterfall in the world — was one of the most popular stops on the Grand Tour. Romantic-era artists such as John Sell Cotman and J.M.W. Turner in Europe, as well as the Hudson River School members in the U.S., painted scenes of waterfalls that increased the public’s thirst for landscape beauty.

A Waterfall Fueled America’s Industrial Revolution

In the 1790s, Alexander Hamilton established the United States’ first planned industrial town around the Great Falls of the Passaic River in New Jersey. Hamilton envisioned the city of Paterson as a hub of American industry, with unlimited hydropower drawn from the falls. Over the next several decades, state-of-the-art mills and factories producing cotton, paper, silk, munitions, and railroad machinery opened, reaching peak productivity in the 1890s. Today, the old manufacturing sites and the waterfall make up the Paterson Great Falls National Historical Park, which honors the city as one of the birthplaces of the American industrial revolution.

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Bunnies are one of the harbingers of springtime, whether they’re feasting in newly green fields or nibbling on freshly planted gardens and the first soft blooms of the year. Some, like the mythical Easter Bunny, even perform the much-welcomed deed of delivering baskets of chocolate or facilitating a large-scale Easter egg hunt. Here are six facts about rabbits to ponder.

Rabbits Live on Every Continent Except Antarctica

More than 29 species of wild rabbits can be found throughout the world, on every continent except for Antarctica. However, one region has an exceptional population: North America is home to nearly half of the world’s rabbits, and the species has developed more biodiversity there than on any other continent. Australia, conversely, doesn’t have any native rabbit species; the European rabbit was introduced by immigrants for sport hunting there in the late 1700s, and today the creatures are considered invasive.

Rabbits and Hares Are Two Different Species

The word “hare” is often used interchangeably with “rabbit,” though it’s not an accurate swap, since rabbits and hares are actually separate species within the Leporidae family. The species have major temperament and anatomical differences, starting from birth. Hares have longer pregnancies than rabbits (about 42 days compared to about 30 days), and their newborns, called leverets, are fully developed when born, meaning they have fur and can open their eyes. Rabbits, on the other hand, are born earlier and have no fur, and they can’t open their eyes until about a week later. Both animals have sleek pelts that shed, but only hares have  dramatically color-changing fur; for example, the snowshoe hare sheds its summertime brown pelt for a white coat as cooler weather arrives, allowing it to blend into snowy ecosystems.

While rabbits can be domesticated as pets or livestock, hares are incredibly skittish and untrusting. In the wild, rabbits live communally in colonies of up to 20 bunnies, and dig extensive underground tunnels called warrens. Hares (which do not burrow) aren’t as sociable, but will group up at mealtimes to safely forage for food. If frightened, hares can move at top speeds, covering distances equal to 37 of their body lengths per second — in comparison, cheetahs, the world’s fastest land animal, only move at 23 body lengths per second.

Rabbits Sleep With Their Eyes Open

Despite being blind for their first week of life, rabbits develop an amazing range of vision. Once they can peek through their eyelids, bunnies have a nearly 360-degree view, which helps them spot predators. Rabbits also have the ability to sleep with their eyes open, because of a nictitating membrane, aka a translucent third eyelid, which keeps the eyes moist.

Beyond their eyes, bunnies rely on two other noteworthy senses — their sound and smell. Rabbits breathe only through their noses, which allows them to smell the world around them, even while eating, and detect danger. Their ears are also incredibly sensitive; rabbits can rotate each ear 180 degrees, picking up on sounds and potential threats up to 2 miles away.

The Word “Bunny” Was Once Used for Both Rabbits and Squirrels

Modern English speakers often refer to rabbits as bunnies, a word that likely came from 16th-century England. Back then, the word “bun” was used in England for both rabbits and squirrels. According to the Oxford English Dictionary, “bunny” appears to have become a more widely used term by the 1700s.

Elsewhere in Europe, rabbits were called “coneys,” from the French word conil, with further roots in the Latin word cuniculus. The term “rabbit” was often used for young coneys, but over time “rabbit” became the more popular word. Today, the word “coney” survives in the place name Coney Island.

One Hare Breed Caused an Adoption Frenzy in the Early 1900s

Beatrix Potter, the Victorian-era author behind The Tale of Peter Rabbit and The Tale of the Flopsy Bunnies, had a love for rabbits and hares — which just so happened to be a popular pet in the late 19th century. Some, like the Belgian hare (which Potter owned and based her Peter Rabbit character on) became so sought-after that adopting one was an extravagant purchase. Belgian hares were first bred in their namesake Belgium, using both domestic and wild hares, and made their way to American rabbit shows in 1877. Between 1898 and 1901, thousands were imported to the U.S. for adoring buyers.

A near-mania called the “Belgian hare boom” emerged, with hares sold at extravagant prices, including a record $5,000 for one creature (about $179,000 today). Around 1900, it was believed that more than 60,000 Belgian hares could be found in Southern California alone. However, like all fads, the Belgian hare market burst when the breed fell out of favor around 1917, and by the 1940s, they were scarcely seen in the U.S. Today, breeders have worked to keep Belgian hares from going extinct.

No One Knows the True Origins of the Easter Bunny

There are many theories as to how bunnies became associated with Easter — like the supposed Anglo-Saxon goddess who turned an egg-laying bird into a rabbit, or how Neolithic communities in Europe buried hares in religious rituals. What is known is that by the 1600s, English hunters specifically sought out hares for Easter meals, possibly linked to a folk tradition thought to scare away witches, who supposedly took the form of hares to cause mischief and illness.

Around the same time, German children celebrated spring’s arrival by receiving gifts from the “Easter hare,” whom they anticipated by making nests for the hare to lay its eggs — possibly the origin of the Easter basket. German immigrants relocating to America brought this tradition with them, and over time it transformed into the chocolate-delivering Easter Bunny many children await each spring.

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Between their serene movement and their permanent smiles, it’s hard to not love sloths. Hanging high up in the trees of Central and South America, both two- and three-toed sloths have captured so many human hearts that some cry at the mere sight of them. (Have you ever seen a baby sloth wearing pajamas? Now you have.)

Because sloths often prefer to keep their distance from us, there’s still a lot to learn about them — but what we do know is fascinating. After all, how many animals have miniature ecosystems in their fur and helped avocados survive? These six facts about sloths will have you running toward your nearest sloth rescue.

Sloths Navigate Mostly By Touch

Sloths don’t have great hearing or eyesight, so they navigate the world primarily by touch using their incredible spatial memory — and they have a keen sense of smell, which helps them find food. Their vision is especially bad; sloths have a condition called monochromacy, meaning they have no cone cells in their eyes at all. This makes them not only colorblind, but mostly blind in dim light and completely blind in bright light. Three-toed sloths can’t even see 5 feet in front of them.

For Tree-Dwelling Animals, They Have a Terrible Sense of Balance

Despite living high up in trees, sloths have little use for balance; they hook themselves onto trees firmly (so firmly, they can sleep suspended), and move very slowly. Since sloths don’t need the same level of motion control as many other mammals, the mechanisms that help a human or a squirrel, for example, find their footing in a tree eroded over generations. When sloths do lower themselves to the ground, usually for their once-per-week trip to the bathroom, they have a lot of trouble moving around gracefully.

Sloths Are Weirdly Good Swimmers

Sloth senses, their musculature, and even their ears have evolved almost perfectly for a narrow set of circumstances: hanging from trees, and moving slowly around trees. On the ground, they’re clumsy and vulnerable. So it might surprise you that they’re actually kind of speedy swimmers. Amazingly, they move three times as quickly in the water as they do in trees. The gas in their stomachs makes them surprisingly buoyant, so all they have to do is paddle those big long arms to cross even wide rivers in the Amazon.

The Three-Toed Pygmy Sloth Is Critically Endangered

Three-toed pygmy sloths are the smallest in both size and population. They’re about 40% lighter than brown-throated sloths, and only became recognized as a distinct species in 2001. Sadly, they are critically endangered, meaning they have an extremely high risk of becoming extinct. Three-toed pygmy sloths started evolving separately from their larger counterparts on Escudo de Veraguas, an island that became isolated from mainland Panama around 9,000 years ago. Researchers still don’t know a lot about their diet, habitat, or even their population — it could be anywhere between 500 and 1,500.

Most sloths are not endangered. However, the maned three-toed sloth, which lives along a small stretch of rainforest coastline in southeastern Brazil, is considered vulnerable.

Sloth Fur Contains an Entire Ecosystem

Each strand of sloth fur contains microcracks, which, along with the creatures’ extremely slow speed, allows algae and fungi (some that aren’t found anywhere else) to grow freely. That algae turns green and creates an extra layer of camouflage for the animals during rainy seasons. Sloth fur is also home to multiple unique species of moths that rely on sloths for survival. When sloths descend to the forest floor for their weekly bathroom break, the moths lay eggs in the dung, which then hatch and fly up to the trees to return to the sloth’s fur. When the insects die and decompose, they fertilize the algae, creating more camouflage and, perhaps, a nutritious snack for grooming sloths.

Gross as it may sound, that dirty fur could hide some medical miracles, thanks to some sloth-exclusive fungi. One researcher found at least 28 distinct strains growing on a three-toed sloth, some with the potential for treating diseases — including breast cancer.

Elephant-Sized Sloths Used To Roam All of North America

Today, sloths seem elusive. They live exclusively in lowland forests in Central and South America, and spend most of their time camouflaged high up in the treetops. However, they were far more commonplace for our human ancestors of the late ice age, who could have encountered sloths the size of elephants as far north as Alaska and the Yukon Territory. One fossil was even found more than 8,000 feet above sea level in the Rocky Mountains.

Large clawed ground sloths (Megalonyx) grew to about 10 feet long and weighed around 2,200 pounds. Shasta ground sloths were a little smaller, and had a much narrower habitat, but were still quite large at 9 feet long and up to 550 pounds. You may have extinct giant sloths to thank for avocados existing, since they were one of a handful of large mammals able to swallow an entire avocado pit and pass it in a new location, allowing more trees to grow. However, humans eventually had to take over cultivation of avocados manually.

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If you’ve ever gotten a face full of spiderweb on an early morning walk, you’re already intimately familiar with one of nature’s most amazing engineering feats. Incredibly strong, super stretchy, and specialized for multiple purposes, spiderwebs are so common that it’s easy to overlook their sophistication. Settle in for a word on webs.

Spiders Have Special Organs to Make Silk for Their Webs

All spiders produce silk, even if most of the world’s roughly 50,000 spider species don’t construct webs. Flexible, durable silk is produced as a liquid protein in organs called spinnerets on the underside of spider abdomens. Each spinneret has at least one spigot that extrudes the protein. When the goo hits the air, it turns into a filament of silk. Many spiders have several kinds of spinnerets, which produce different types of sticky and nonsticky silk.

Spiderwebs Are Extremely Versatile

Think of spiderwebs as tiny nets to catch specific meals. Flying insects get trapped in nearly invisible webs that are oriented vertically, while horizontal webs catch ants, caterpillars, and other non-airborne morsels. Most spider species kill the prey stuck in their webs with a venomous bite. One odd group is an exception: Instead of disabling with venom, spiders in the family Uloboridae wrap their prey in an enormous amount of silk, boa constrictor-style, that eventually crushes it to death. An orb weaver spider named Philoponella vicina has been observed wrapping its victims in more than 260 feet of silk.

Spiders can also use their silk like a lasso to proactively grab a meal, as a parachute to travel through the air, as a temporary shelter during inclement weather, and even as an airtight scuba tank when diving underwater.

Spiderwebs Are Amazingly Strong

Spider silk is often described as being, pound for pound, five times stronger than steel. In a 2018 study in the journal ACS Macro Letters that examined brown recluse spiders’ silk, scientists explained how the tiny threads of a spiderweb achieve their strength: Each strand is made of thousands of even tinier nanofibers stuck together in a ropelike form, making the whole more durable. The nanofibers also make the strands flexible and resistant to breaking. Spiderwebs can absorb the impact of a bug flying into them, for example, by stretching upon initial contact and then strengthening under continued pressure, localizing any damage while preserving the integrity of the web.

Spiderwebs Come in a Variety of Shapes

Spiders in the family Araneidae spin orb webs — the classic spiderweb shape that’s circular and geometrically segmented with spokes radiating from the center. These often span open spaces between tree branches or in the corners of a building. Spiders in the family Linyphiidae build sheet webs horizontally over a patch of grass or foliage; the thickly woven web forms a cover for the spider to hide under while it waits for prey to approach.

Tangle webs (also called cobwebs) are built by spiders that belong to the family Theridiidae, which includes black widows. These webs look like a messy clump of threads supported by a solid base, such as a rock or tree branch. Funnel weaver spiders, in the family Agelenidae, build webs with a large horizontal opening and a tube-shaped funnel in the center or on the side, where the creatures wait for prey to wander by and become stuck in the filaments. Funnel webs are usually found near the ground amid brush or grass.

Designers Have Made Clothes From Spider Silk

People have been trying to adapt light, flexible spider silk for clothing for a long time. In 1709, the French naturalist François Xavier Bon de Saint Hilaire succeeded in making gloves and stockings from spider silk painstakingly collected by boiling egg sacs and teasing out the fibers. In the late 19th century, French Jesuit priest Paul Camboué invented an apparatus to extract silk from golden orb weaver spiders, but his captive arachnids cannibalized each other — a problem that later spider silk farmers also encountered. As a result, spider silk clothing is extremely difficult — but not impossible — to make. Designers Simon Peers and Nicholas Godley, plus a large team of fiber artisans, managed to create a cape and shawl made from golden orb weaver silk and displayed the items in 2012 at London’s Victoria and Albert Museum.

Scientists have also tried to bioengineer spider silk and mimic its strength and beauty. In 2015, The North Face and a materials company called Spiber created the “moon parka,” a jacket with an outer shell of synthetic spider silk manufactured by genetically engineered microbes.

Spiderwebs Can Heal Wounds

If you have a cut on your arm, a spiderweb would probably be the last thing you’d think to put on your skin, but ancient Greeks often placed them over wounds to stop bleeding. The proteins that form silk are high in vitamin K, a coagulant that aids blood clotting. One Roman surgeon also suggested getting rid of a wart by wrapping it in a spiderweb and setting it on fire (don’t try this at home). French soldiers in the 14th-century Battle of Crécy carried spiderwebs in their uniforms to serve as bandages, and Shakespeare later cited the webs’ styptic quality. In A Midsummer Night’s Dream, Bottom says to the fairy Cobweb, “I shall desire you of more acquaintance, good Master Cobweb/If I cut my finger I shall make bold with you.”