Planet Earth is home to some spectacular relics from bygone eras, constructions that seem to defy the technological capabilities of their time either because they’re too big, too heavy, or too complex.

As such, some suggest the ancient builders of the Egyptian pyramids, the Nasca lines, and others were following an extraterrestrial instruction manual. Perhaps the hands that crafted these sites weren’t really of this world.

To be sure, it’s fun to think about whether aliens have visited Earth. After all, humans are on the threshold of expanding our reach in space, and places like Mars are in our sight. But the truth is, there’s no evidence suggesting that aliens have ever been here. And invoking a supernatural explanation for some of the most monumental of human achievements means skipping over the fascinating ways in which prehistoric civilizations managed to make some of the largest and most enigmatic constructions on Earth.

Sacsayhuamán

Outside the old Inca capital of Cusco, a fortress called Sacsayhuamán rests in the Peruvian Andes. Built from enormous stones that have been chiseled and stacked together like a jigsaw puzzle, some say Sacsayhuamán could be the work of an ancient civilization that had a little help from interstellar friends.

The 1,000-year-old interlocking fortress walls are made of rocks that weigh as much as 360 tons each, and which were carried more than 20 miles before being lifted and fit into place with laser-like precision.

How an ancient culture accomplished such a feat of engineering is a fun little problem to solve; turns out the Inca were as adept at building houses and fortified complexes as they were at watching the sky and keeping calendars. In fact, Sacsayhuamán isn’t the only example of this intricate masonry: Similar walls exist throughout the Inca Empire, including one in Cusco where a 12-angled stone has been carefully wedged into place.

More recently, archaeologists have uncovered traces of the rope-and-lever system the Inca used to transport stones from their quarries to their cities—a system that relied on strength and ingenuity, rather than alien architects.

Nasca Lines

On a high and dry plateau some 200 miles southeast of Lima, more than 800 long, straight white lines are etched into the Peruvian desert, seemingly at random. Joining them are 300 geometric shapes and 70 figures of animals, including a spider, monkey, and hummingbird.

The longest of the lines run straight as an arrow for miles. The biggest shapes stretch nearly 1,200 feet across and are best viewed from the air. Scientists suspect the Nasca drawings are as many as two millennia old, and because of their age, size, visibility from above, and mysterious nature, the lines are often cited as one of the best examples of alien handiwork on Earth. Otherwise, how would an ancient culture have been able to make such huge designs in the desert without being able to fly? And why?

Turns out, it’s rather easy to understand the how. Called geoglyphs, these enigmatic designs are made by removing the top, rust-colored layer of rocks and exposing the brighter white sand underneath.

The why is a bit tougher to comprehend. First studied in the early 1900s, the designs were initially suspected to be aligned with constellations or solstices, but more recent work suggests the Nasca lines point to ceremonial or ritual sites related to water and fertility. And in addition to being visible from the air, the shapes can be seen from surrounding foothills.

Egyptian Pyramids

Just outside Cairo, in Giza, the most famous of Egypt’s pyramids rise from the desert. Built more than 4,500 years ago, the Pyramids at Giza are monumental tombs where ancient queens and pharaohs were buried.

But how, exactly, did the Egyptians build these things? The Great Pyramid is made of millions of precisely hewn stones weighing at least two tons each. Even with today’s cranes and other construction equipment, building a pyramid as big as that of Pharaoh Khufu would be a formidable challenge.

And then there’s the astronomical configuration of the pyramids, which is said to align with the stars in Orion’s belt. As well, alien theorists often point to the fact that these three pyramids are in way better shape than others built centuries later (never mind the amount of work that has gone into preserving them over the past several centuries).

So are Egypt’s pyramids artifacts of aliens? Not exactly. It’s true that scientists aren’t quite sure how the ancient Egyptians build the pyramids—and especially how they did it so quickly—but there’s ample evidence that these tombs are the work of thousands of earthly hands.

Stonehenge

A huge circle of stones, some weighing as much as 50 tons, sits in the English countryside outside Salisbury. Known as Stonehenge, the Neolithic monument inspired Swiss author Erich von Däniken to suggest it was a model of the solar system that also functioned as an alien landing pad—after all, how else could those massive stones have ended up hundreds of miles from their home quarry?

No one knows what, exactly, the meaning of Stonehenge is, but, as with all the other sites in this collection, the explanation is not aliens. Instead, scientists have demonstrated it’s actually possible to build such a thing using technologies that would have been around 5,000 years ago, when the earliest structures at the site were built.

And now, it appears as though the stones are aligned with solstices and eclipses, suggesting the Stonehenge builders were at least keeping an eye on the heavens, even if they didn’t come from above.

Teotihuacán

Teotihuacán, meaning the "City of the Gods," is a sprawling, ancient city in Mexico that’s best known for its pyramidal temples and astronomical alignments. Built more than 2,000 years ago, Teotihuacán’s age, size, and complexity can make it seem otherworldly, but it’s very much the work of humans.

Scientists suspect that over centuries, a mix of cultures including Maya, Zapotec, and Mixtec built the city that could house more than 100,000 people. With its murals, tools, transportation system, and evidence of advanced agricultural practices, Teotihuacán is often considered much more technologically developed than should have been possible in pre-Aztec Mexico.

By far, the most well known of Teotihuacán’s buildings is the massive Pyramid of the Sun. One of the largest such constructions in the Western Hemisphere, the pyramid’s curious alignment is believed to be based on calendrical cycles.

Easter Island

The enigmas surrounding the moai, Easter Island’s fleet of large stone figures, pretty much follow the same narrative as the other sites described here: How in the world did the Rapa Nui make these figures more than 1,000 years ago? And how did the moai end up on Easter Island?

Carved from stone, the nearly 900 human figures are sprinkled along the flanks of the island’s extinct volcanoes. The figures average 13 feet tall and weigh 14 tons and appear to have been chiseled from the soft volcanic tuff found in the Rano Raraku quarry. There, more than 400 statues are still in various states of construction, with some completed figures awaiting transportation to their intended resting place.

The reasons for carving the moai are mysterious, though they were likely sculpted for religious or ritual reasons. It’s also not exactly clear what happened to the stone-crafting Rapa Nui, but a leading theory suggests their civilization succumbed to an environmental disaster of their own making … which is something that probably could have been prevented had ancient aliens bestowed their infinite wisdom upon the culture.

The Face on Mars

If Elon Musk has his way, humans will be capable of visiting the “face on Mars” sometime this century. Spotted by the Viking 1 orbiter in 1976, the so-called face is nearly two miles long and is in a region called Cydonia, which separates the smooth plains of the Martian north from the more cratered terrain in the south. At the time, scientists dismissed the “face” as shadow play, but over the decades it has become a favorite among those who suspect aliens with a penchant for building things have been visiting the solar system.

In 2001, NASA’s Mars Global Surveyor took another good look at the face—using a much higher resolution camera—and saw … no face. Turns out that what had appeared to be a face is just another boring old Martian mesa, kind of like the landforms that litter the U.S. Southwest.

But that doesn’t mean it wouldn’t be fun to visit.