Makronissos is a rock small island of the Cyclades in the west of Kéa and the south of the Laurion. It has a surface of 15 km ² and a littoral of 28 km. All in length, it makes 3 km length on 400 to 500 m broad.

In antiquity, the small island would have been called Helene, because the beautiful one would have stopped there, either while going, or while returning from Troy. Archaeological evidence shows that the small island was inhabited as of prehistory.

It is in the channel separating Makronissos and Kéa that the ship hospital Britannic , twin of the Titanic ran, on November 21st, 1916, touched by a mine with 8:02 of the morning. It touched the bottom of the Aegean Sea later 55 minutes. It rests by 120 m basic. The shipwreck made about thirty deaths. More than 1000 people were initially saved share the fishermen of Korissia, village of Kéa. The ship then did not transport of casualties; it went to the face Dardanelles.

During the Greek Civil war and the Dictatorship of the colonels, Makronissos was used like place of deportation of the political opponents, mainly the communist . Nowadays uninhabited, the small island is regarded as a memorial of the civil war.