The Pentland Firth (Scottish Gaelic: An Caol Arcach, meaning the Orcadian Strait), which is actually more of a strait than a firth, separates the Orkney Islands from Caithness in the north of Scotland.
The name is presumed to be a corruption of the Old Norse "Petlandsfjörð", meaning "the fjord of Pictland", and is completely unrelated to the Pentland Hills near Edinburgh.
Prior to the Norse occupation of Orkney the strait was known as the "Sea of Orcs" - a reference to the Pictish tribe who inhabited Orkney. Such was their marine prowess that there are also instances of this name referring to the sea lanes of the entire west coast of Scotland down to Kintyre. One version of the 9th century Historia Brittonum states that "the Britons originally filled the whole island with their peoples from the English Channel to the Sea of Orcs".<ref>Watson (1994) 62-63</ref>
On the Caithness (southern) side the Firth extends from Dunnet Head, in the west to Duncansby Head in the east, while on the Orkney (northern) side from Tor Ness on Hoy in the west to Old Head on South Ronaldsay in the east.
In the middle of the Firth are two significant islands, Stroma and Swona. The small Pentland Skerries group are in the east. The islands of Hoy and South Ronaldsay border the firth to the north and are part of the Orkney Islands.
The most northerly point of the headland of Dunnet Head, Easter Head, is also that of mainland Britain. The famous John o' Groats, Mey (site of the Castle of Mey), and many smaller villages are also to be found on the Caithness side as is the town of Thurso and Scrabster Harbour in Thurso Bay, on the western fringe of the Firth.
Crossing the Firth
In the West the ferry from Scrabster to Stromness operated by NorthLink is the oldest continuous ferry service across the firth by the ferry MV Hamnavoe, started in 1856 as a continuation of the railhead at Thurso.
Historically the Gills Bay area has been the main setting off point from the mainland to the islands of Stroma and Swona and Orkney itself. This route, known as The Short Sea Crossing, is both the quickest and safest route across the stormy waters of the Pentland Firth. At present Pentland Ferries operate on this route from Gills Bay to St Margaret's Hope on South Ronaldsay.
At the Eastern end John o' Groats Ferries sail to Burwick, also on South Ronaldsay. This is a small passenger ferry and is run in the summer months only.
Consideration was also given to construction of a tunnel linking the islands to the mainland. The 10 mile tunnel was initially projected to have cost £100 million based on preliminary studies carried out in 2005 but as of 2012 no further progress has been made.