Kythnos (Greek: Κύθνος) is a Greek island and municipality in the WesternCyclades between Kea and Serifos. It is 56 nautical miles (104 km) from the harborof Piraeus. Kythnos is 100 km2 (39 sq mi) in area and has a coastline of about 100 km (62 mi). It has more than 70 beaches, many of which are still inaccessible by road. Of particular note is the crescent-shaped isthmus of fine sand at Kolona.