Pentland Firth was known as the "Sea of Orcs".
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 Firth is well known for the strength of its tides, being among some of the fastest in the world, a speed of 16 knots (30 km/h) being reported close west of Pentland Skerries. The force of the tides gives rise to overfalls and tidal races which can occur at different stages of the tide.
Seals and porpoises can be seen all year round in the firth. Dolphins are not very common, but seen from time to time. Orcas are seen occasionally as are minke whales. Basking sharks are rare but being seen more often around May to August.
The Firth was first swum across by Colleen Blair in August 2011. Blair has previously crossed the North Channel between Scotland and Ireland and completed several other open water swims of note including the English Channel and circumnavigation of Jersey and Manhattan Island.