Stonehenge has long been a mystery to archeologists. Researchers have excavated, x-rayed, measured and analyzed the standing stones, but scientists are still in the dark as to what the stones are for and why they were built in the first place.

There have been numerous theories proposed for why this 5,000 year-old monument was erected. Some scholars say it helped mark time. Another theory puts Stonehenge as a place of worship. Another recent idea is that the monument is a center of healing. Researchers found new evidence that could unlock the mystery of this enigmatic monument.

Archeologists now posit another idea: that Stonehenge was a ritual site long before the standing stones were put in place. Researchers led by the University of Birmingham along with the Ludwig Boltzmann Institute for Archaeological Prospection uncovered two giant pits to the east and west of Stonehenge, giving evidence of a pre-existing ritual site.

"These exciting finds indicate that even though Stonehenge was ultimately the most important monument in the landscape, it may at times not have been the only, or most important ritual focus, and the area of Stonehenge may have become significant as a sacred site at a much earlier date," Vince Gaffney, an archaeology professor at the University of Birmingham, said in a statement.

The pits were discovered as part of the Stonehenge Hidden Landscape Project, initiated last year with backing from the IBM Visual and Spatial Technology Center. The pits which measure about 5 meters wide and at least 1 meter deep can't easily be seen from the ground. They were found on the Stonehenge Cursus, an immense enclosure about 100 meters wide and 2.5 kilometers across north of Stonehenge.

The Birmingham team theorized that the pits may have contained tall stones or even fires to mark the rising and setting sun. The Cursus may have been a central stage for a midsummer festival that was enacted long before Stonehenge was constructed.

"It is possible that processions within the Cursus moved from the eastern pit at sunrise, continuing eastwards along the Cursus and, following the path of the sun overhead, and perhaps back to the west, reaching the western pit at sunset to mark the longest day of the year. Observers of the ceremony would have been positioned at the Heel Stone, of which the two pits are aligned," Gaffney said.

Observers standing at the Heel Stone, the massive upright stone outside Stonehenge's main circle, would have seen the sun rise above the eastern pit on the summer solstice. The same observer would also see the sun set that evening in line with the western pit.

Gaffney adds that the pits also suggest that Stonehenge may not have been the only ritual site in the area. The area might have been significant long before Stonehenge. Gaffney estimates that there are about two more years of survey work to do in the area. Even then researchers might not fully uncover all the mysteries of Stonehenge.