Earth is covered with incredible geological structures, from volcanos to crystal-encrusted caves to awe-inspiring canyons. While some of our planet’s mysteries have been solved, some of its formations defy easy explanation. Here are a few that continue to baffle scientists.

1. The Eye of the Sahara // Mauritania

ISS Crew Earth Observations Facility and Earth Science and Remote Sensing Unit, Johnson Space Center, NASA // Public Domain

The Eye of the Sahara, also known as the Richat Structure, is a 28-mile-wide site of huge concentric circles found in the western African nation of Mauritania. Geologists initially thought the site was created by an asteroid impact, but there isn’t enough melted rock among the rings to support this theory. Similarly, there’s no evidence to suggest a volcanic eruption. New Age enthusiasts hint that the Eye of the Sahara could represent the remains of the mythical sunken island of Atlantis, based on Plato’s allegory.

More recently, geologists have proposed that the Eye of the Sahara could be an eroded, collapsed geological dome, formed some 100 million years ago when the supercontinent Pangea broke up. Bolstering this theory are ancient rocks found on the surface, which originated as much as 125 miles beneath the Earth’s crust and before life existed on Earth. Research continues.

2. Lake Hillier // Australia

Kurioziteti123, Wikimedia Commons // CC BY-SA 4.0

This small, saltwater lake on an island off Western Australia is only one-third of a mile long, but its bubblegum-pink color makes it especially striking. The lake was documented in 1802 by British explorer Matthew Flinders, who took a sample of its waters but failed to understand how it got its startling hue. Tourists can visit only by helicopter, though it is safe to swim in the waters.

Scientists today suspect the color is due to the presence of a pink alga, Dunaliella salina, and/or a pink bacterium, Salinibacter ruber. But unlike other pink lakes around the world, such as Lake Retba in Senegal, Lake Hillier’s color doesn’t fluctuate with temperature or sunlight—so the investigation goes on.

3. The Great Unconformity // United States

Alex Demas, USGS // Public Domain

The Great Unconformity is a huge gap in the geological record: Layers of rock dating from about 1.2 billion to 250 million years ago are completely missing from certain areas around the globe. This enormous chunk of lost time can be seen clearly in the stratigraphy of the Grand Canyon in Arizona. Geologists studying the anomaly there have noted that there is plenty of rock, full of fossils, from the Cambrian period (540 million years ago) but the layer beneath it is basement rock, formed roughly 1 billion years ago and empty of fossils. So, what happened to the stuff in between?

An emerging theory—"Snowball Earth”— may explain where the rock disappeared to. Around 700 million years ago, Earth was encased in snow and ice. Moving glaciers peeled off the planet’s crust with the help of lubricating sediments, pushing it into oceans, where it was reabsorbed by subducting tectonic plates. Many questions remain unanswered, though—such as the multimillion-year gap between the end of Snowball Earth, around 635 million years ago, and the start of the Cambrian period.

4. Nastapoka Arc // Canada

Jeff Schmaltz, MODIS Land Rapid Response Team, NASA GSFC, Wikimedia Commons // Public Domain

In the southeast corner of Hudson Bay, Canada, lies a near-perfect arc. The mysterious half-circle, also known as the Hudson Bay Arc, was first thought to be an impact crater from a meteorite. But none of the usual confirming evidence, such as shatter cones or unusual melted rocks, has been found in the vicinity.

The most commonly accepted theory for the arc, based on geological evidence collected in the 1970s and later, is that it is a boundary formed when one shelf of rock was pushed under another other. That doesn’t explain how or why is it’s so perfectly round—so the Nastapoka Arc remains subject to ongoing study.

5. Mima Mounds // United States

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The Mima Mounds are mysterious, uniform undulations in the grasslands of Washington State near Olympia, ranging from 10 to 164 feet in diameter and up to 6.5 feet tall. When American explorer Charles Wilkes set eyes on them in 1841, he believed they were human-made burial mounds and had three of them excavated, only to find them filled with loose stones. Similar mounds are found from California to Colorado and have puzzled naturalists for years.

Scientists suggest that some of the mounds may be 30,000 years old, which makes decoding them complex; humans are believed to have arrived in North America several thousand years later than that. Many theories about their cause—glacial flooding, whirlpools, and even wind-blown sediment clumping around vegetation—have been dismissed. The current leading theory, based on computer modelling, is that pocket gophers created the mounds. Yet doubts remain: No one has ever witnessed a pocket gopher building one.

6. Fairy Circles // Namibia

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Up close, the fairy circles in the Namib Desert are just circular patches of bare red earth, surrounded by tufts of grass. But from a bird’s-eye view, these spots stretch endlessly across the arid landscape, creating a regular polka-dot pattern. Folktales claim the spots are the gods’ footprints, but scientists have searched for an evidence-based explanation.

At first, some proposed that the circles are created when plants compete for water: The root systems of the successful vegetation dominate the ground, while smaller plants are unable to compete, leaving bare patches of desert. In 2017, a promising new theory appeared in the journal Nature. Excavations of several circles revealed termite nests under each one, implying the circles were created by the termites eating the vegetation above their territory, allowing desert grasses to flourish only between each nest. Ecologists modeled both the plant-competition and hungry-termite theories, and found that both supported conditions conducive to fairy circles. But with such a complex ecosystem, scientists say more research is needed.

7. Yamal Craters // Russia

Jesse Allen, Earth Observatory, NASA // Public Domain

In 2014, a helicopter pilot flying over the Yamal Peninsula in Siberia, which juts into the Kara Sea, noticed an enormous hole in the permafrost. Scientists rushed to analyze the nearly 100-foot-wide crater and determine its origin. A meteorite impact, a natural gas explosion, or alien interference were all floated as possible causes.

Tests of the air at the bottom of the crater revealed very high levels of methane, pointing to an explosion—possibly brought on by several unusually warm summers that destabilized the permafrost. But an equally likely explanation, according to some researchers, is that the crater represents a slow, long-term collapse of the permafrost itself rather than a recent explosion. Since then, more craters have been discovered. Further study is needed, but the treacherous permafrost makes research difficult.