Stonehenge has mystified people for centuries, but it was largely agreed the Altar Stone - one of the biggest rocks in the prehistoric structure - had originally come from Wales.

But now - with the help of Australian researchers - scientists have discovered it actually hails from Scotland, and may have been transported by sea - not land.

• Guest:Anthony Clarke, Australian from Curtin University who is part of the international team of geo-chronologists, currently in the UK
• Producer: Kate Jones