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What’s the Difference Between a Stalactite and a Stalagmite?

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Well-meaning geologists ended up confusing plenty of folks when they named stalactites and stalagmites. Both of these similar-sounding structures—typically formed in limestone caves—are capable of stretching over 27 feet in length. But what's the difference between them, and how do such strange ornaments grow in the first place?

Let’s clear up the terminology. Here’s a helpful (and widely-used) phrase you can use to sort out which is which: “Stalactites hold tight to the ceiling and stalagmites might touch the ceiling.” In other words, stalactites form on the roofs of caves and dangle downward like rocky icicles. Stalagmites, in contrast, are based on the floor and stretch upwards, only occasionally coming into contact with the overhanging ceiling.

An alternate memorization method goes as follows: “stalactite” is spelled with a “t,” as in “top.” “Stalagmite” uses the letter “g," as in “ground.”

According to the Oxford English Dictionary, these two terms descended from the Greek word stalaktos, which means “to drip." This is because trickling rainwater is responsible for the conical objects’ formation. When rain seeps through limestone, the water extracts carbon dioxide gas from the rock. What results is a weak carbonic acid that penetrates the stone and deposits a patch of calcite on the caves’ roof. As the water continues dripping, more and more calcite is added to the spot, eventually producing a lengthy stalactite.

But what about stalagmites? There’s a reason these are generally found directly underneath stalactites—all that dripping water has to land somewhere, after all. When a drop finally hits the cave floor, it deposits even more calcite there, this time in an unassuming mound. The liquid keeps dripping off the tip of a stalactite and the lump keeps rising, leaving us with a surging stalagmite. To call this process gradual would be a gross understatement. In limestone caverns, the usual growth rate is under 10 centimeters per millennium. 

It’s also worth noting that both stalagmites and stalactites belong to a larger geological group known as “speleothems." This is an extensive family of differently-shaped mineral formations that also includes globular “cave popcorn” and stunningly-beautiful “flowstones." Additionally, lava is occasionally involved in stalactite creation, leading to some strange-looking results. 

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Big Questions
Why Is the American Flag Displayed Backwards on Military Uniforms?
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In 1968, famed activist Abbie Hoffman decided to crash a meeting of the House Un-American Activities Committee in Washington by showing up in a shirt depicting the American flag. Hoffman was quickly surrounded by police, who ripped his shirt off and arrested him for desecration of the Red, White, and Blue.

Hoffman’s arrest is notable today because, while it might be unpatriotic to some, wearing the American flag, burning it, or otherwise disrespecting it is not a violation of any federal law. In 1989, the Supreme Court ruled that it would be unconstitutional to prosecute any such action. Still, Americans have very fervent and strict attitudes toward displaying the flag, a longstanding symbol of our country’s freedom. According to the U.S. Flag Code, which was first published in 1923, you shouldn’t let the flag touch the ground or hang it upside-down. While there’s no express prohibition about reversing the image, it’s probably a safe bet you shouldn’t do that, either.

Yet branches of the U.S. military are often spotted with a seeming mirror reflection of the flag on their right shoulder. If you look at a member in profile, the canton—the rectangle with the stars—is on the right. Isn’t that backwards? Shouldn’t it look like the flag on the left shoulder?

The American flag appears on a military uniform
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Not really. The flag is actually facing forward, and it’s not an optical illusion.

When a service member marches or walks forward, they assume the position of a flagpole, with the flag sewn on their uniform meant to resemble a flag flapping in the breeze. With the canton on the right, the flag would be fluttering behind them. If it were depicted with the canton on the left, the flag would be flying backward—as though it had been hung by the stripes instead of the stars nearest to the pole. The position of the flag is noted in Army Regulation 670-1, mandating the star field should face forward. The official term for this depiction is “reverse side flag.”

As for Hoffman: His conviction was overturned on appeal. In 1970, while at a flag-themed art show in New York, he was invited to get up and speak. He wore a flag shirt for the occasion.

Have you got a Big Question you'd like us to answer? If so, let us know by emailing us at bigquestions@mentalfloss.com.

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Big Questions
What Causes Sinkholes?
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Mark Ralston/AFP/Getty Images

This week, a sinkhole opened up on the White House lawn—likely the result of excess rainfall on the "legitimate swamp" surrounding the storied building, a geologist told The New York Times. While the event had some suggesting we call for Buffy's help, sinkholes are pretty common. In the past few days alone, cavernous maws in the earth have appeared in Maryland, North Carolina, Tennessee, and of course Florida, home to more sinkholes than any other state.

Sinkholes have gulped down suburban homes, cars, and entire fields in the past. How does the ground just open up like that?

Sinkholes are a simple matter of cause and effect. Urban sinkholes may be directly traced to underground water main breaks or collapsed sewer pipelines, into which city sidewalks crumple in the absence of any structural support. In more rural areas, such catastrophes might be attributed to abandoned mine shafts or salt caverns that can't take the weight anymore. These types of sinkholes are heavily influenced by human action, but most sinkholes are unpredictable, inevitable natural occurrences.

Florida is so prone to sinkholes because it has the misfortune of being built upon a foundation of limestone—solid rock, but the kind that is easily dissolved by acidic rain or groundwater. The karst process, in which the mildly acidic water wears away at fractures in the limestone, leaves empty space where there used to be stone, and even the residue is washed away. Any loose soil, grass, or—for example—luxury condominiums perched atop the hole in the ground aren't left with much support. Just as a house built on a weak foundation is more likely to collapse, the same is true of the ground itself. Gravity eventually takes its toll, aided by natural erosion, and so the hole begins to sink.

About 10 percent of the world's landscape is composed of karst regions. Despite being common, sinkholes' unforeseeable nature serves as proof that the ground beneath our feet may not be as solid as we think.

A version of this story originally ran in 2014.

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