A mysterious prehistoric tomb on the island of Orkney has a special "light box" cut into its roof, archaeologists have discovered. It allowed a shaft of light to herald the start and end of winter.

This has only been seen once before and it may literally cast new light on the lives and beliefs of primitive Neolithic man.

Hidden since the stone age

The Crantit tomb was discovered last year when a tractor disturbed a series of flat stones just below the surface. These turned out to be the roof slabs of an underground tomb, hidden below a hillside for over 5,000 years.

The Glasgow University archaeological field unit directed by Dr Beverley Ballin Smith and Dr Colin Richards were called in and soon realised the importance of the important find.

The small tomb consists of three chambers in a "clover-leaf" formation and was almost invisible from above ground.

It is highly unusual as most chambered cairns in Orkney and elsewhere were built on the surface as prominent mounds.

"It was a totally unexpected find," Dr Ballin Smith told BBC News Online.

Looking toward the entrance

During the excavation one of the archaeologists, a skilled stonemason, noticed something unusual about the slabs of stone that formed the roof.

"What we found was truly remarkable," said Dr Ballin Smith.

A notch had been cut in the roof to allow a ray of sunlight to penetrate the tomb, but only at certain times of the year.

In October and again in February, at the beginning and the end of what was no doubt a harsh winter, the Sun would have thrown a shaft of light along the length of the tomb.

It is the first tomb found in the UK that has a light shaft. The only other one known is at Newgrange in Ireland.

Mysterious patterns

Strange carvings were found on the upright stone pillar that holds up the roof.

"If you look closely you can see geometric patterns and symbols carved into the rock," Dr Ballin Smith said. "We think that they may even have been coloured with primitive paints and pigments."

No human remains were found in the tomb's central chamber but bones from four individuals were found in side chambers: a mature woman, a girl, a child and one too degraded to identify. Fragments of pottery were also found but no ornaments.

Neolithic tombs were frequently used over and over again, but entrance to the Crantit tomb was heavily blocked up, both inside and out, suggesting reuse was not wanted.

"Perhaps it tells us something about a cultural change in the treatment of death and burial five thousand years ago," speculated Dr Ballin Smith.