Controversial tunnel under Stonehenge approved over archaeologists' objections

Revelers gather to observe the summer solstice sunrise. Thousands flock annually to the site to welcome the longest day of the year in the Northern Hemisphere.

Sunlight bursts through a trilithon at Stonehenge, located on England's Salisbury Plain. Built on an ancient earthen structure, the monument was constructed from bluestones brought from a distance of more than 150 miles. Larger sarsen stones are believed to have been brought from Marlborough Downs, about 19 miles away.

Scholars and scientists have long searched for the meaning of Stonehenge, and theories about its origins abound. The way the gaps in the monument's concentric rings align with the sun—at dawn on the summer solstice and at sunset on the winter solstice—have led some to speculate that its primary purpose was astronomical.

Stonehenge was added to the World Heritage List in 1986, along with Avebury—the world's largest prehistoric stone circle—and other Neolithic sites in the area. Here, sheep graze the plain near a burial mound, or barrow.

Henges are described as circular banks of earth paralleled by an internal ditch. Stonehenge itself is not, strictly speaking, a henge: The position of its bank and ditch are reversed.

In 2014, scientists announced that sophisticated underground mapping techniques had revealed an astonishing complex of ancient monuments, buildings, and barrows that had been hidden beneath the Stonehenge area for thousands of years.

Archaeologically significant sites around Stonehenge include the Avenue, the Cursus, and Durrington Walls. The area is also home to Britain's densest concentration of round barrows.