Carregal do Sal in Portugal is home to mysterious graves that may have once acted like a giant telescope for people of the ancient past. As bizarre as this may seem, this is not the only known megalithic observatory from long ago. So, what makes this massive stone structure in Portugal unique compared with similar structures, such as Stonehenge in England and the Pyramids of Giza in Egypt?

Carregal do Sal consists of a network of passage graves, similar to several others found scattered around Europe, dating back 6,000 years, to the New Stone Age. They are constructed with long, narrow hallways, backed by a flat surface. New analysis of passage graves found in Portugal reveals the means by which the structures may have served as telescopes for ancient people.

Stonehenge is constructed from large stone slabs, buried into the ground, and rising above the land by 15 feet or more. Like the passage graves of Portugal, these ancient pillars are designed to align with objects in the sky. Such an arrangement would allow our distant ancestors to know when to plant certain crops, or move livestock to new grounds for grazing.

Ancient astronomical tools such as Stonehenge and the Pyramids of Giza utilize placements to create easy views of celestial alignments critical to ancient people. However, the passage graves in Portugal take this idea a step further. Some of the best views of the sky can be seen from within the tomb itself. While contemporary telescopes collect and focus light, the passage graves would have allowed a person to see just a small portion of the sky at a time, and would have blocked extraneous light from the sun at dusk or dawn. In near pitch-dark conditions, the eyes of the observer would also become accustomed to the dark, allowing the viewer to see fainter stars than would otherwise be possible.

"[T]he orientations of the tombs may be in alignment with Aldebaran, the brightest star in the constellation of Taurus. To accurately time the first appearance of this star in the season, it is vital to be able to detect stars during twilight," Fabio Silva of the University of Wales Trinity Saint David said.

The Pyramids of Giza and Stonehenge offer no such adaptations, meaning the graves of Portugal would have been more advanced than some better-known ancient observatories.

However, the construction of Stonehenge may remain a greater mystery. The stones, weighing up to 25 tons each, were transported around 20 miles from Marlborough Downs to the Salisbury Plain, before the invention of the wheel. Some of the smaller stones, weighing as much as 4 tons, may have been brought to the site from 140 miles away from their current location.

"Stonehenge is perhaps the world's most famous prehistoric monument. It was built in several stages: the first monument was an early henge monument, built about 5,000 years ago, and the unique stone circle was erected in the late Neolithic period about 2500 BC. In the early Bronze Age many burial mounds were built nearby," English Heritage reports.

Carregal do Sal may have been the Hubble Space Telescope of the Neolithic Age.

Photo: Adriano Aurelio Araujo | Flickr