Though ancient standing stones, stone circles, and megaliths are scattered across the world, scientists and historians continue to debate their purposes. Theories about their significance abound. Some propose they served as astronomical sites, while others suggest past civilizations built them as places of ritual and worship. For centuries, people have also puzzled over how the heavy stones were transported and erected long before the creation of the wheel, let alone other modern technology. Here are six megalithic sites still shrouded in mystery.

1. Stonehenge // England

Stonehenge in Salisbury, England, is one of the most iconic megalithic sites in the world. Neolithic people began building the circle of stones around 3000 years ago. The blue stones that make up the ancient monument have been traced to two specific quarries in Pembrokeshire, Wales, hundreds of miles from where the circle now stands. Historians have theorized that the stones were transported to England on rafts down rivers, then pulled on wooden sleighs using rollers, a process that must have involved months of hard work. The stone circle is just part of a series of ancient structures strewn across the landscape of Salisbury Plain. The site has long been a sacred space—even today, pagans gather there to celebrate on the winter and summer solstices.

2. Carnac // France

Karsten Wentink, Wikimedia Commons // CC BY-SA 4.0

Around the small village of Carnac in Brittany, France, stand thousands of ancient menhirs (single upright monoliths) and other types of megaliths. The rows of stones have been dated to the Middle Neolithic period (around 3000 BCE), but an exact date has yet to be proposed. There are over 3000 of them, measuring as much as 20 feet high and stretching for a total of more than 4 miles. The site includes groupings of megaliths, burial mounds, and enclosures, representing an extraordinary feat of Neolithic construction. The arrangements are long thought to have served some ritual or religious purpose, but no one is quite sure what. Historians studying the site have proposed that the lines of stones delineate a sacred space, perhaps leading people toward an area of worship. According to one popular legend, when the Roman army was marching on Brittany, the wizard Merlin appeared and turned the soldiers into the stones.

3. Mysterious Stone Spheres // Costa Rica

Archaeologists have debated the exact origin and purpose of these stones. Though excavations around some of the spheres revealed pre-Columbian pottery, because so many of the mysterious orbs have been damaged or moved from their original sites, pinpointing their origin and date of creation has become difficult. Some of the stones were discovered in seemingly astronomically significant alignments, leading some archaeologists to propose they may have been astronomical or navigational tools.

What we know for sure is that the stones are made of hard igneous (solidified from lava or magma) rock such as granodiorite. They were shaped by humans rather than nature. You also don’t have to head to Costa Rica to see the strange spheres, as two were transported to the U.S. One stands at the National Geographic Museum in Washington, D.C., and the other is in a courtyard near Harvard’s Peabody Museum of Archaeology and Ethnography in Cambridge, Massachusetts.

4. Deer Stones // Mongolia and Siberia

Richard Mortel, Flickr // CC BY 2.0

The deer stones are a series of more than 1200 ancient standing stones scattered across Mongolia and Siberia, given their name because many of them include elaborate carvings of flying deer. The stones range in height from about 3 to 13 feet and are often grouped together. Scientists believe Bronze Age nomads erected them over 3000 years ago. Though depictions of deer are most prevalent, the structures also feature carvings of elk, people, and representations believed to be the sun and moon. The intricate carvings would have taken a lot of skill, so historians think the stones may have been dedicated to great warriors or chiefs.

5. Avebury Henge // England

Michael Dibb, Geograph // CC BY-SA 2.0

You’ll find the world’s largest prehistoric stone circle in the quaint village of Avebury, not far from Stonehenge. The circle originally contained some 100 megaliths and encircled two smaller stone rings. The stones are believed to form part of a wider ritual landscape, which was built and altered from about 2850 to 2200 BCE. Archaeologists think the circles, henges, and avenues of stones formed part of a public space for religious ceremonies, but their exact use and the nature of those ceremonies remain a mystery.

In the 1930s, an excavation by archaeologist Alexander Keiller revealed a grisly surprise: a skeleton crushed beneath one of the stones. The body did not belong to one of the Neolithic builders, but rather a man from the 14th century who was killed when trying to move one of the megaliths.

6. Gochang Dolmen Site // South Korea

jeong woo Nam, Flickr // CC BY-NC-ND 2.0

This huge prehistoric burial site in South Korea is spread across the areas of Gochang, Hwasun, and Ganghwa. It contains hundreds of ancient dolmens—tombs built from large stone slabs. These Neolithic and early Bronze Age structures are made from two or more stones, topped with a large capstone to form a marker for a burial site. The sheer number of dolmens is the most surprising aspect of this World Heritage Site, with thousands dotted across the Korean landscape. It’s the highest concentration of dolmens in the world.