Some were built in the shapes of animals, serpents, and bears, their outlines most visible from the sky.
Others were designed to align with the sun during summer and winter solstices, key markers of the changing seasons for people who depended on stars and landmarks for navigation.
Together they represent some of the earliest man-made monuments on the planet — earthen mounds built by ancient native architects across the eastern woodlands of the United States.
They number in the thousands. And their mystery has long obsessed American archaeologists, including Megan Kassabaum, the Weingarten assistant curator for North America at the University of Pennsylvania Museum.
Arduous digging and research has failed to explain essential enigmas:
Why did roving bands of hunter-gatherers devote precious time and energy to building mounds, only to move on and build again somewhere else?
Why did Native Americans build so many mounds, hundreds of thousands of them, many later destroyed by farming and development?
And why mounds? Why choose a shape defined by the absence of edges and corners?
Beginning Saturday, the Penn Museum takes on those and other questions in an exhibit that analyzes 5,700 years of mound-building through videos, photographs, and artifacts.
"I want people to come away with a greater appreciation for the deep history of their country, and a better understanding of the variety and complexity of native cultures that existed and continue to exist," said Kassabaum, an archaeologist who directs the museum's Smith Creek Project in Mississippi.
She curated the exhibit, which runs through December.
For many early tribes, mound-building became a central, vital activity. It was as if they couldn't build enough of them. The largest required epic feats of engineering and sweat, the moving of tons of earth without metal tools or the wheel.
Mounds were built in different eras for different and often multiple purposes — as burial chambers, to host religious rites, or as foundations for important buildings. They mostly appeared near rivers and tributaries. The oldest are older than the pyramids, older than Stonehenge, making wonders such as the Roman Coliseum and Tower of London seem like historical greenhorns.
To modern American Indians, the mounds are holy, and they can see excavation as desecration.
"The mounds should be treated as sacred," said Ruth Cankudutawin Hopkins, a native activist and author of War Bonnets, Bikinis and Genocide: Exploring Pop Culture's War Against Native Ethos.
Though more constraints are in place now, archaeological digs have excavated human remains and delivered precious relics to museum shelves.
Thomas Jefferson famously directed the 18th-century excavation of a mound near Monticello in Virginia. In Philadelphia, physician Montroville Wilson Dickeson, an accomplished amateur archaeologist, undertook digs in Louisiana and Mississippi and in the 1850s promoted his lectures in Barnumesque tones.
"Diagrams, Paintings, and upward of Ten Thousand Specimens of American Antiquities," crowed a handbill.
In the late 1920s, native skeletons at Dickson Mounds in Illinois were put on public display. Last year a bill that would have let Wisconsin landowners excavate and develop Indian mounds was halted after loud protests.
"It's essentially grave-robbing," Hopkins said. "Society and historically, scientists as well as anthropologists tend to conduct themselves as though American Indians are extinct."
Hopkins said her children's great-great-great grandfather Chief Bluedog is buried in a mound near Enemy Swim Lake in South Dakota. His descendants still visit his grave.
Kassabaum doesn't excavate burial mounds. When she inadvertently comes across human remains, she said, she tries to talk to the governing tribe about whether and how to proceed.
Her work focuses on mound-building in the Lower Mississippi River Valley, home to thousands of mounds in different sizes and forms.
Near St. Louis, Cahokia Mounds is among the largest, most complex pre-Colombian archaeological sites north of Mexico. At its peak in 1200, Cahokia encompassed 120 mounds and 15,000 people, making it larger than many cities in Europe.
Its 100-foot tall Monk's Mound required millions of baskets of dirt, all hauled by hand.
At Cahokia stands perhaps the most enigmatic mound, known as Mound 72. Within it were found the remains of several elites — and about 270 others, more than half of whom had been ritually sacrificed.
The skeletons of four young men were missing their hands and heads. A mass grave held more than 50 young women.
Today, Cahokia is an official Illinois historic site, complete with guided tours and a gift shop.
But smaller, lesser-known mounds can be found across the East, Midwest and South. Nearest to Philadelphia is a mound in Irvine, Warren County, southeast of Erie, and another in the baseball mecca of Cooperstown, N.Y.
"You don't need a passport to visit extraordinary, ancient monuments," Kassabaum said.
To a layman, a smaller mound can appear as ordinary as a hill in the woods, or a raised ring of lawn on a golf course. More would be revealed if it was possible to see inside.
"They're not just piles of dirt," Kassabaum said. "They're elaborately constructed. But it's hard to see that from the outside when they're covered in grass."
She thinks the early mounds might have been territorial markers, providing a kind of home base, something familiar for hunter-gatherers who moved with the seasons. The dome shape could reference the Earth, the construction a reenactment of the world's origin story.
It's possible, she said, that the building process was as important as the mounds themselves — a coming together of community. One way to think of that, she said, is like a prehistoric barn-raising.
It's plain from alignments and artifacts that mound-builders possessed deep knowledge of the sun, moon and stars. "These people understood the world better than us in some ways," Kassabaum said.
Why so many mounds? Why wasn't 1,000 or 10,000 enough? One answer may be that the mounds were like churches: Every community and denomination wanted its own.
Mound-building slowed and then stopped abruptly around 1500. That's roughly the time of contact, when European violence and disease decimated tribes.
The exhibit explores that, too. And how mound-building has been resurrected in a community in North Carolina, where the Eastern Band of Cherokee Indians tend the ancient Kituwah mound, revered as the birthplace of their people.
"Initially, I was drawn in by the mystery that surrounds them," Kassabaum said. "Today I recognize just how much we do know, and yet they haven't become any less interesting."