Although it’s one of the world’s most famous monuments, the prehistoric stone circle known as Stonehenge remains shrouded in mystery. Built on Salisbury Plain in Wiltshire, England, Stonehenge was constructed in several stages between 3000 and 1500 B.C., spanning the Neolithic Period to the Bronze Age.

Its massive scale suggests that Stonehenge was vitally important to the ancient peoples who built it, but the monument’s purpose has been the subject of widespread speculation for centuries. In the 17th and 18th centuries, many believed Stonehenge was a Druid temple, built by those ancient Celtic pagans as a center for their religious worship. Though more recent scholars have concluded that Stonehenge likely predated the Druids by some 2,000 years, modern-day Druidic societies still see it as a pilgrimage destination.

One enduring hypothesis for Stonehenge’s purpose comes from the initial observation, first made by 18th-century scholars, that the monument’s entrance faces the rising sun on the day of the summer solstice. For many, this orientation suggests that ancient astronomers may have used Stonehenge as a kind of solar calendar to track the movement of the sun and moon and mark the changing seasons.

New excavations in recent years, however, have unearthed a different theory based on hundreds of human bones found at the site, dating across 1,000 years and showing signs of cremation before burial. The presence of these remains suggests that Stonehenge could have served as an ancient burial ground as well as a ceremonial complex and temple of the dead.

In 2010 archaeologists discovered a second stone circle located just over a mile away from the more famous landmark. Dubbed “Bluestonehenge” for the 25 Welsh bluestones that originally made up the site, this secondary monument provides more evidence that Stonehenge could have been part of a huge memorial complex where high-ranking individuals took part in elaborate rituals and ceremonies honoring the dead. Yet as no written records exist, this theory—like all those about Stonehenge’s purpose—can only remain a matter of speculation.