The Grand Canyon is known for its dramatic landscape and the long geological record visible in its rock walls. That record contains a notable gap, however, which has puzzled geologists for more than 150 years. Known as the Great Unconformity, the gap represents a period when about one billion years of rock is missing from the canyon’s geological sequence.
A team of geologists from the University of Colorado has proposed an explanation. They suggest that the missing rock was washed away into the ocean during the breakup of the ancient supercontinent Rodinia around 700 million years ago. The researchers used thermochronology, a method that studies the heating history of rocks, during an expedition to the canyon. Their findings indicate that the eastern and western halves of the canyon may have experienced different geological changes over time. This suggests the upheaval affected each half differently and at slightly different times, which may account for the gap. The research also provides insight into similar geological patterns at other North American sites.
Several other features of the Grand Canyon are notable. The elevation ranges from roughly 2,000 feet to over 8,000 feet, creating significant temperature differences between the top and the bottom. The canyon contains more than a thousand caves, though only 335 have been documented and a smaller number fully explored. One cave, the Cave of the Domes on Horseshoe Mesa, offers a visitor suite that can be reserved after the park closes. Supai Village, located within the Havasupai Indian Reservation near the canyon, is a remote community of about 200 people that receives mail by pack mule.
Some rock in the canyon is about 1.75 billion years old, making it among the oldest exposed rock on the planet. The Colorado River continues to erode the canyon walls, a process that will eventually reshape landmarks such as the Horseshoe Bend.
The canyon system on Mars, known as Valles Marineris, is far larger than the Grand Canyon, stretching more than 2,500 miles. It is visible from Earth through a telescope. Scientists believe the canyon formed after the crust of Mars cracked and fractured due to upheaval in the Tharsis region, where several large volcanoes are located, including Olympus Mons. Features within the canyon, such as collapse pits and seeping patterns, suggest water was present at some point in the planet’s history. Valles Marineris also contains volcanic cones dating from 200 to 400 million years ago.
Archaeological discoveries have also revealed several ancient cities that were unknown for centuries. In Mexico, LiDAR technology helped identify Guiengola, a Zapotec city covering 360 hectares with over a thousand buildings. It was inhabited between 1350 and 1521 and was built into mountain slopes, with residential areas, temples, and defensive walls. The Zapotec people successfully defended the city against the Aztecs before relocating to a nearby settlement with better access to farmland. About 300,000 Zapotec people still live in Oaxaca today.
LiDAR has also revealed a network of settlements in the Amazon rainforest. These include rectangular platforms, roads, and canals arranged around central plazas, suggesting that the region supported complex urban societies earlier than previously thought.
In Turkey, a man renovating his home in 1963 discovered an entrance to Derinkuyu, an underground city in Cappadocia. The settlement extends roughly 279 feet deep and was large enough to shelter about 20,000 people with livestock and supplies. It included chapels, storage areas, and a ventilation shaft. The Phrygians are believed to have built the earliest levels, with later use by the Romans and Byzantines.
Off the coast of Egypt, the submerged city of Thonis-Heracleion was discovered in 2000. The ruins included 64 ships, 700 anchors, and gold coins. The city served as Egypt’s main port for international trade before it was covered by water, likely after a flood near the end of the second century BCE.
The site of Vichama in Peru, excavated over eight years, is associated with the Caral-Supe civilization, one of the oldest known societies in the Americas. It existed from roughly 3000 to 1800 BCE and left behind large structures, including stepped pyramids and ceremonial plazas. No evidence of conflict has been found among Caral sites, which suggests a largely peaceful society. The causes of the civilization’s decline are not certain; possible explanations include climate change or internal organizational difficulties.
In Belize, archaeologists working at the Maya city of Caracol found the tomb of a ruler named Te’ K’ab Chaak, dated to around 1,600 years ago. The tomb contained jade beads, shell jewelry, pottery, and a jade mosaic mask. The ruler is considered the founder of a dynasty that lasted more than 460 years. Caracol was one of the largest Maya cities, covering about 77 square miles at its peak with an estimated population of 100,000. The tomb was hidden beneath another burial discovered in 1993, which preserved it from looters.
The Inca maintained a network of suspension bridges made from grass, with about 200 in use across ravines. The Q’eswachaka Bridge in Peru is still rebuilt each June by local villagers. The Inca also built stone walls in Cusco using blocks fitted together without mortar. These structures have withstood earthquakes because the stones can shift slightly during tremors and settle back into place. Researchers using ground-penetrating radar have identified a network of underground tunnels beneath Cusco, possibly connecting the Coricancha temple to the fortress of Sacsayhuamán. The tunnels are estimated to be about eight to nine feet wide and five feet tall, built using a cut-and-cover method.
The Amazon rainforest is home to a wide range of wildlife, including pink dolphins, piranhas, anacondas, and the Goliath birdeater spider. The region also contains plants such as the Victoria amazonica water lily and the socalled walking palm tree, which can move short distances over time in search of sunlight. About 30 million people live in the Amazon, and recent research suggests that up to one million people may have inhabited the region between 1250 and 1500 CE. LiDAR surveys in the Bolivian Amazon have revealed urban centers connected by causeways and canals, with some settlements built by the Casarabe culture between 500 and 1400 CE. These communities may have been affected by drought, though the exact cause of their decline is unknown.
The legend of El Dorado originated from Muisca ceremonies in which a leader covered in gold dust entered a lake as an offering. European explorers interpreted this as evidence of a city rich in gold and organized expeditions to find it. Most returned empty-handed or did not return at all. In the early 1900s, the British explorer Percy Fawcett became convinced that a lost city, which he called Z, existed in the Amazon. He disappeared during an expedition in 1925 along with his son and a companion. Later discoveries of large earthworks in southern Amazonia have supported the idea that complex societies existed in the region, although no city matching Fawcett’s description has been found.
The Amazon also contains unusual geographical formations. Cerro El Cono in Peru is a symmetrical isolated peak near the Brazil border, which some have speculated could be a lost pyramid. Geologists generally consider it a volcanic cone or a product of erosion. Indigenous communities in the region regard such mountains as sacred protectors. The Chocolate Hills in the Philippines are another distinctive formation, consisting of more than a thousand limestone mounds that turn brown during the dry season. They were formed by the erosion of marine limestone over millions of years.
Several underwater archaeological sites have provided information about past human activity. In Lake Huron, researchers found obsidian flakes beneath 100 feet of water on a land bridge that once connected Michigan and Ontario. The obsidian originated in Oregon, indicating long-distance trade networks roughly 10,000 years ago. The site also preserved ancient trees and peat bogs. Off the coast of Greece, Pavlopetri is considered the oldest known submerged city, dating back about 5,000 years. Near Japan, the Yonaguni Monument consists of stepped stone formations about 82 feet underwater. Some researchers believe it may be the remains of an ancient civilization, while geologists argue it is a natural rock formation shaped by tectonic activity and erosion.
In Lake Michigan, divers found a stone circle that includes a carving of a mastodon, an animal extinct for about 10,000 years. Scientists have proposed that the structure may have been used for hunting or had ceremonial significance. Lake Michigan is also the site of numerous shipwrecks, including the Margaret A. Muir, which was located in 2024 after sinking 130 years earlier. In Lake Mendota, Wisconsin, researchers found 11 ancient canoes, the oldest dating to around 2500 BCE. And in Lake Constance in Switzerland, archaeologists have identified a line of 170 man-made stone structures dating back about 5,500 years.
Puma Punku, part of the Tiwanaku site in Bolivia, is notable for its precisely cut stone blocks, some weighing up to 131 tons. Researchers do not know how the builders moved and shaped the blocks, which were sourced from miles away. The Tiwanaku people thrived between roughly 500 and 1000 CE. A UCLA research team created a 3D model of the site and found that its design resembles structures at other nearby sites. They concluded that the construction was the work of human builders rather than extraterrestrials.
The Serapeum of Saqqara in Egypt is a burial complex containing large granite sarcophagi that held the remains of sacred Apis bulls. Some of the boxes weigh around 70 tons. An inscription indicates that moving a single sarcophagus took 28 days. The purpose of the oversized boxes remains uncertain, though some have suggested they may have served a different technical function.
Several other archaeological mysteries remain unresolved. In Sardinia, statues of 38 human figures standing about eight feet tall were assembled from broken fragments in 2005. They were created by the Nuragic people, who also built thousands of stone structures on the island. In South Africa, skull fossils known as Boskop Man suggest a species that may have had larger brains than modern humans, though the interpretation is debated.
The island nation of Malta is home to megalithic temples built around 3500 BCE, older than the Egyptian pyramids. Some stones weigh as much as 20 tons. Local legend attributes the construction of one temple to a giant woman named Sunsuna. Excavations at the Hypogeum, an underground burial complex, have produced elongated skulls whose origin remains uncertain. The Ħal Saflieni Hypogeum and other temple sites, such as Mnajdra, show astronomical alignments with the sun during solstices and equinoxes.
