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Mysterious ‘Sailing Stones’ Explained

It might just be the world’s slowest game of Red Light, Green Light: Huge rocks in Death Valley National Park race across the mud flats—but only when nobody is looking. The sailing stones, as they’re called, mystified park visitors and scientists for decades. But one man believes he’s solved the puzzle.

The dry lakebed known as the Racetrack playa looks much like the rest of Death Valley—cracked and dry in the summer and frozen in winter. But nowhere else in the park will you find strange tracks trailing behind each big rock and boulder. It looks as though the stones have dragged themselves through the desert, or as though they have been dragged, by some great cosmic hand. 

All summer, the stones are still. Through autumn, they don’t budge. Then winter roars in and creeps out. By spring, the stones have moved again.

Natural and supernatural theories have abounded: Unsurprisingly, many attributed the rocks’ stealthy movements to aliens. Some said wind was the culprit; others ice; others rain; still others, mystical energy fields. Some people even steal rocks from the park, hoping to harness their magical powers. 

Scientists have set up experiments in the playa since the 1940s, trying to understand what makes the sailing stones sail. But all the results have been inconclusive, and despite frequent checks, nobody has ever been able to catch the rocks in motion.

It took a space researcher to crack the case. Planetary scientist Ralph Lorenz was working with NASA, setting up miniature weather stations in Death Valley, when he first became interested in the stones. (Conditions in the park are so severe that it’s often used as an experimental stand-in for Mars.) Although his original work was focused on summer in the desert, Lorenz realized his instruments would work just as well for monitoring the playa’s rocks in the winter. Lorenz and his team pored over images of the stones’ trails, looking for some clue.

One rock stood out. “We saw one instance where there was a rock trail and it looked like it hit another rock and bounced, but the trail didn't go all the way up to the other rock, like it was repelled somehow,” Lorenz told Smithsonian.com. “We thought if there was a collar of ice around the rock, then it might be easy to imagine why it might bounce.” 

The final breakthrough came not in the laboratory or even in the desert, but in the kitchen. Lorenz poured a little water in a plastic container, then dropped in a small rock and put the whole thing in the freezer. When he took it out, he had a rock half-sheathed in ice. That rock went into another dish, this one filled with water atop a layer of sand. He set the rock in the water, ice side up, and blew on it to give it a gentle push. The rock floated across the water, scraping a path through the sandy bottom as it went. Lorenz had found the answer. 

“Basically, a slab of ice forms around a rock, and the liquid level changes so that the rock gets floated out of the mud,” he said. “It’s a small floating ice sheet which happens to have a keel facing down that can dig a trail in the soft mud.” 

Lorenz published his findings in the American Journal of Physics in 2011. Once he knew what to look for, he was even able to catch the rocks in motion.

Not everybody is satisfied with the ice-raft explanation. Park visitors ask why it happens, but they don’t want to hear about science, park ranger Alan Van Valkenburg told Smithsonian.com. “People like a mystery—they like an unanswered question.”

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science
Geological Map Shows the Massive Reservoir Bubbling Beneath Old Faithful
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Yellowstone National Park is home to rivers, waterfalls, and hot springs, but Old Faithful is easily its most iconic landmark. Every 45 to 125 minutes, visitors gather around the geyser to watch it shoot streams of water reaching up to 100 feet in the air. The punctual show is one of nature’s greatest spectacles, but new research from scientists at the University of Utah suggests that what’s going on at the geyser’s surface is just the tip of the iceberg.

The study, published in the journal Geophysical Research Letters, features a map of the geological plumbing system beneath Old Faithful. Geologists have long known that the eruptions are caused by water heated by volcanic rocks beneath the ground reaching the boiling point and bubbling upwards through cracks in the earth. But the place where this water simmers between appearances has remained mysterious to scientists until now.

Using 133 seismometers scattered around Old Faithful and the surrounding area, the researchers were able to record the tiny tremors caused by pressure build-up in the hydrothermal reservoir. Two weeks of gathering data helped them determine just how large the well is. The team found that the web of cracks and fissures beneath Old Faithful is roughly 650 feet in diameter and capable of holding more than 79 million gallons of water. When the geyser erupts, it releases just 8000 gallons. You can get an idea of how the reservoir fits into the surrounding geology from the diagram below.

Geological map of geyser.
Sin-Mei Wu, University of Utah

After making the surprising discovery, the study authors plan to return to the area when park roads close for the winter to conduct further research. Next time, they hope to get even more detailed images of the volatile geology beneath this popular part of Yellowstone.

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Animals
Why Do Female Spotted Hyenas Give Birth Through Their Pseudo-Penises?
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At the zoo, you can sometimes tell the difference between male and female animals by noting their physical size, their behavior, and yes, their nether regions. Hyenas, however, flip the script: Not only are lady spotted hyenas bigger and meaner than their male counterparts, ruling the pack with an iron paw, they also sport what appear to be penises—shaft, scrotum, and all.

"Appear" is the key word here: These 7-inch-long phalluses don't produce sperm, so they're technically really long clitorises in disguise. But why do female hyenas have them? And do they actually have to (gulp) give birth through them? Wouldn't that hurt … a lot?

The short answers to these questions are, respectively, "We don't know," "Yes," and "OW." Longer answers can be found in this MinuteEarth video, which provides the full lowdown on hyena sex. Don't say we didn't warn you.

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