1. Stonehenge was built in phases.

Around 3000 B.C. a circular earthwork was constructed at the site, consisting of a ditch (dug using tools made from antlers) with an inner and outer bank. Inside the bank were 56 pits, which became known as the Aubrey Holes, after antiquarian John Aubrey, who identified them in 1666. Archaeologists estimate Stonehenge was home to 150 or more cremation burials from approximately 3000 B.C. to 2300 B.C., and they’ve called it Britain’s biggest known cemetery of the time.

The two types of stones at the center of the monument, the large sarsens and smaller bluestones, arrived at the site sometime around 2500 B.C. Afterward, they were shaped using various stoneworking techniques and arranged in formations. The final stage of construction was a ring of pits now referred to as the Y holes, dug sometime between 1600 B.C. to 1500 B.C. The Y holes encircled another ring of pits called the Z holes, which were dug at an earlier time and surrounded the sarsens. Researchers are unclear as to whether the Y and Z holes served any purpose. It’s also unknown how long Stonehenge continued to be used after the Y holes were dug.

2. It’s a mystery how some stones got to the site.

Among the remaining riddles about Stonehenge is how its builders, who had only primitive tools, managed to haul all the massive stones to the site. The sarsen stones, which each weigh an average of 25 tons, are thought to have been brought to the site from Marlborough Downs, about 20 miles to the north. The bluestones, which weigh between 2 tons to 5 tons, were transported to Stonehenge from the Preseli Hills area in West Wales, a distance of more than 150 miles. Most archaeologists believe that humans moved the bluestones over water and land to Stonehenge, although it’s also been suggested these stones could’ve been pushed to the site by glaciers.

In 2000, a Welsh group called Menter Preseli attempted to use only Stone Age tools and methods to recreate the prehistoric journey made by the bluestones. The project involved dragging a bluestone weighing across land on a large wooden sled then transporting it over water by boat. However, various problems arose along the way, including the theft of the sled (it was soon found but a crane was needed to get the stone back on it). Later, as the stone was being carried in a sling between two long rowboats, it fell into the water and sank (after divers located the stone, it had to be raised by a salvage crew). Eventually, the entire project was scrapped.

3. Stonehenge once was put up for auction.

Starting in the Middle Ages and for centuries afterward, Stonehenge was privately owned. By the late 1800s, crowds of visitors had taken a toll on the site. Sir Edmund Antrobus, owner of the land on which Stonehenge is situated, resisted calls from preservationists to sell the property to the British government. In the early 1900s, Antrobus’s son put up a fence around the prehistoric monument and for the first time in its history visitors were charged an admission fee. Meanwhile, the British military began establishing training facilities in the surrounding area, resulting in an influx of soldiers, equipment and, eventually, aircraft, some of which crashed near the site. However, the passage of the Ancient Monuments Consolidation and Amendment Act in 1913 protected Stonehenge from being intentionally demolished. In 1915, after the Antrobus family heir was killed during World War I, Stonehenge went up on the auction block, where local resident Cecil Chubb successfully bid on the site, on a whim, for £6,600. Three years later, Chubb donated Stonehenge to the national government. In recognition of this deed, he was knighted by Prime Minister Lloyd George.

4. Theories abound about Stonehenge’s purpose.

Stonehenge’s builders left no known written records, so scholars (and non-scholars) have long speculated about why it was constructed. In the early 12th century, Geoffrey of Monmouth, one of the first people to write about the ancient site, claimed it was erected as a memorial to hundreds of Britons who were slayed by the Saxons. According to Geoffrey, the wizard Merlin supposedly directed that the stones for the monument be procured from the Giants’ Ring, a stone circle with magical healing powers said to be located in Ireland. Another theory, suggested by John Aubrey and 18th century archaeologist William Stukeley, is that Stonehenge was built as a Druid temple. Modern scholars say Stonehenge’s construction predated the Druids; however, present-day Druids view it as a sacred spot.

Another theory, introduced in the 1960s, holds that Stonehenge was an astronomical computer used to predict eclipses. And in 2008, archaeologists suggested that Stonehenge was a center for healing, a prehistoric version of Lourdes that attracted the sick and injured. Meanwhile, there’s a contingent of people who believe Stonehenge is a landing pad for ancient space aliens, and British authorities have received reports from the public about UFOs hovering near the famous monument.

5. Summer solstice gatherings were banned at Stonehenge.

First held in 1974 during the summer solstice, the Stonehenge Free Festival started as a counter-culture gathering that grew significantly in size over time. After tens of thousands of people showed up for the 1984 festival, authorities, concerned about such issues as open drug use, banned the event for the following year. Nevertheless, on June 1, 1985, a long convoy of vehicles filled with would-be festival goers (who were part of a movement called the New Age Travellers) made its way toward Stonehenge.

About seven miles from the ancient site, police stopped the convoy. Accounts of what happened next vary: Law enforcement officers claimed they were attacked by people in the vehicles, while those in the convoy said the police dragged various individuals, unprovoked, from their vehicles and beat them. The Travellers fled to a nearby beanfield, where they were surrounded by police, and more violence ensued. Two dozen people were hospitalized, and numerous arrests were made. In the aftermath of the so-called Battle of the Beanfield, summer solstice gatherings at Stonehenge were prohibited until 2000.

6. Darwin studied worms there.

In 1877, naturalist Charles Darwin traveled to Stonehenge to conduct research on a subject that had long fascinated him: earthworms. During his visit, Darwin, who was interested in the impact that worms had on objects in the soil over time, observed how a fallen stone at the ancient monument had sunk deeper into the ground as a result of the activities of the lowly creatures, who continually churn through the soil. Darwin’s research was included in what would be his final book, “The Formation of Vegetable Mould Through the Action of Worms,” published in 1881.

7. Stonehenge is just one of several prehistoric stone circles in Great Britain.

While Stonehenge has been referred to as the most architecturally sophisticated ancient stone circle, the largest of them is Avebury, located 25 miles north of Stonehenge. Constructed between 2850 B.C. and 2200 B.C., Avebury today consists of a massive circular bank and ditch enclosing 28.5 acres. Inside the ditch is an inner stone circle that encloses two smaller stone circles. During the Medieval era, a number of the stones were knocked over and buried by local Christians who believed they were pagan symbols. Later, some of the stones were broken up and used as building materials. In the 1930s, archaeologist Alexander Keiller, heir to a British marmalade fortune, purchased the site. Keiller cleared away old cottages and farm buildings and re-erected many of the stones. As with Stonehenge, it’s uncertain for what exact purpose Avebury was used by ancient people.