Suspected for a long time. Solomon had his tin mines in Tarsus. Tarsus was considered the end of the world. (Jonah was fleeing from God to Tarsus.) So now there is evidence that Britan is Tarsus. Kind of cool.
Tarsus was a city in Anatolia. He didn't have mines there. Jonah was fleeing to Tarshish, see one of the early posts in this topic.