The Orkney Islands are an archipelago of over 70 islands, of which 20 are inhabited, located north mainland Scotland. The largest island is known as the Mainland, the largest settlement and the administrative centre is Kirkwall. Orkney has been inhabited for at least 5,500 years. The first inhabitants were neolithic tribes, then the Picts. In 875 Orkney was invaded and finally annexed by Norway and settled by the Norse. In 1472 it was annexed to the Scottish Crown, after the failed payment of a dowry agreement.
The Orkneys are like Scotland the remains of an old orogeny, with many crystalline and metamorphic rocks. There is no soluble rock, as a result there is no karstification and no karst caves. There are some sea caves along the coasts, but no caves of special touristic interest. But there are numerous subterranea, as Orkney contains some of the oldest and best preserved Neolithic sites in Europe.
The earliest known settlement is at Knap of Howar, a 5,500 years old Neolithic farmstead on the island of Papa Westray. Skara Brae, inhabited around 3,100 BC, is Europe's best preserved Neolithic settlement. There are passage graves, standing stones, brochs, a sort of round towers, and the typical underground houses called weems.