A large rock
in the Atlantic Ocean
, about 500 km west of the Scottish mainland, which makes it the westernmost and most isolated part of Britain. The name is from the Gaelic Sgeir Rocail
No-one cared about it until the 1950s, when the prospect of its commanding a large part of the sea containing valuable oil deposits led to Britain sending a really titchy expeditionary force to claim it. Ireland, Iceland, and Denmark also claimed it but lost out. It was declared British territory in 1955 and incorporated into the United Kingdom in 1972.
This was commemorated in a Flanders and Swann song, about the decline of the Empire, ending with the marvellous lines "In fact, we found... Rockall".
The Wolfe Tones also wrote a song about this, "Rock on Rockall", in which they say the Irish tradition is that it was a pebble that Finn MacCool threw into the sea.
It is uninhabited. It is just a rock. A great big granite* rock about 20 m high, but a rock, not an island. In 1975 an SAS man spent forty days there. In 1997 Greenpeace occupied it and put in a claim for it, after having spent more than 40 days on it, in order to prevent commercial exploitation of the oil fields. See http://www.gpuk.org/atlantic/library/gallery/rockall.html for pictures of them occupying it, and a nice red sunset.
Rockall is also one of the regions of the shipping forecast, bounded by Bailey (to the north), Hebrides, Malin, and Shannon (to the south). There is also a Rockall Trough and a Rockall Bank to the west of Ireland.
The name Rockall has also been used for a fictitious country on a very big island in the North Atlantic, in what seems to be an e-novel at http://www.lights.com/rockall, with plenty of history and geography worked out for it.
* If in deference to the write-up below it is actually basalt, then Flanders and Swann made the same error: or at least, "granite" was a convenient rhyme for "gannet".