America is full of natural sights that look almost impossible at first glance. Some glow, some move on their own, and some appear only at a certain time of year. Here are nine of the strangest, along with the simple science that explains each one.
1. Sailing Stones, Death Valley
In the dry lake bed of Racetrack Playa in Death Valley, heavy rocks leave long trails in the mud, as if they moved by themselves. For many years, no one had ever seen them move. Scientists now know that on rare cold nights, a thin sheet of ice forms on the playa, and when it breaks up and the wind pushes the ice panels, the rocks are slowly dragged along. The trails are left behind in the soft mud once the ice melts. It is a slow and quiet process, but it explains a mystery that puzzled visitors for decades.
2. Firefall, Yosemite National Park
Each year in late winter, Horsetail Fall in Yosemite can look like a waterfall of glowing lava. This happens only when the conditions are just right: the waterfall must be flowing, the sky must be clear, and the setting sun must strike the falling water at exactly the right angle. The sunlight turns the water a bright orange color for a few minutes. It is not fire at all, just light, water, and perfect timing.
3. Grand Prismatic Spring, Yellowstone National Park
Grand Prismatic Spring is the largest hot spring in Yellowstone, and it is famous for its rainbow-like rings of color. The vivid colors come from heat-loving microbes that live in the water around the spring. Different types of microbes thrive at different temperatures, so the cooler outer edges show different colors than the hot center. The deep blue center is so hot that almost nothing can live there, and its color comes from the depth and clarity of the water itself.
4. Bioluminescent Waves, California Coast
On some nights, waves along the California coast glow an electric blue as they break on the shore. The glow comes from tiny ocean organisms called dinoflagellates, a type of plankton. When these organisms are disturbed by moving water, they produce light through a chemical reaction. The effect is strongest during blooms, when huge numbers of them gather near the coast. The glow is a natural defense that can startle predators, and for beach visitors it turns an ordinary wave into a glowing one.
5. Aurora Borealis, Alaska
The northern lights are curtains of green, purple, and pink light that dance across the night sky in Alaska. They are caused by charged particles from the sun that travel to Earth and interact with gases high in the atmosphere. When these particles collide with the gases, the gases release energy as light. Alaska is one of the best places in the United States to see the aurora because it lies under the auroral oval, the ring-shaped zone where the lights appear most often. Clear, dark winter nights give the best views.
6. Morning Glory Pool, Yellowstone National Park
Morning Glory Pool is a small hot spring in Yellowstone named for its shape and colors, which reminded early visitors of a morning glory flower. Its rings of yellow, orange, and green come from heat-loving microbes living at different temperatures around the pool. The center is a deep, clear blue because the water there is very hot and deep. Over time, changes in the spring have cooled parts of the pool, which has allowed more microbes to grow and shifted its colors. It remains one of the most photographed springs in the park.
7. Green Flash at Sunset, Florida Keys
The green flash is a brief flash of green light seen just as the sun disappears below the horizon. It is caused by the way Earth's atmosphere bends, or refracts, sunlight. As the sun sets, the atmosphere acts like a prism and separates sunlight into colors, and the green light is the last to be seen before the sun drops out of view. The flash lasts only a second or two, so it is easy to miss. Flat, clear horizons like those in the Florida Keys make it easier to spot.
8. Lenticular Clouds, Mount Rainier, Washington
Lenticular clouds are smooth, lens-shaped clouds that often hover near mountain peaks like Mount Rainier, sometimes stacked in layers. They form when moist air is pushed up over a mountain and then flows down the other side, creating standing waves in the air. Clouds form at the crests of these waves, where the air cools, and disappear in the troughs, where it warms again. Because the wave stays in place, the cloud can look frozen in the sky even though air is moving through it. Their flying-saucer shape has sometimes been mistaken for something far stranger.
9. Moonbow, Cumberland Falls, Kentucky
A moonbow is a rainbow made by moonlight instead of sunlight. At Cumberland Falls in Kentucky, moonbows appear on clear nights around the full moon, when moonlight shines through the mist rising from the waterfall. The light is bent and split by water droplets in the same way sunlight creates a daytime rainbow. Moonbows are usually fainter than rainbows and may look white to the eye, though a camera can reveal their colors. Cumberland Falls is one of the few places in the world where moonbows appear regularly.
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