- Makronisos lies on the western end of Cyclades. It is only 2nm far from the coasts of Attica and falls under the authority of the Municipality of the neighboring Kea.
- It took its current name after its elongated shape (from north to south) with has a length of 13km.
- In late antiquity (Strabo times) it was named Cranae in honor of the daughter of the mythic king of Athens, Cranaus. As it is identified to the homonymous Homer islet, where Paris and Helen made a stop on their way to Troy, it is also called “Helen’s island”.
- Makronisos covers an area of 18.4km and its coastline has a total length of 37km.
- It has three bays at Agkaliastro, Gerolimnionas and Mavria.
- Its highest peak at the north end of the island is the hill of Tripiti (281m) which has a natural tunnel.
- There is great natural cave at Katazygia.
- The soil is rocky and barren with only a few springs that always dry up in the summer.
- Mild winters, warm summers, strong winds, lots of sunshine and drought are the main characteristics of the climate.
- The subsoil of the island is mostly relevant to Lavrio rather than to the rest Cycladic islands. It is made of limestone and shale layers, with many visible argentiferous ores (galena). Several underground mining undertakings for argentiferous ores have been identified in many areas in Makronisos. Some are attributed to the antiquity and others to modern times.
- Vegetation is dominated by Mediterranean maquis. Moorlands and phygana fields cover the largest area of Makronisos.
- The land is solely a state property. It is periodically inhabited by fishermen, farmers and beekeepers of Lavrio or technicians sent by the Ministry of Culture for repair works in the buildings.