Kythnos has more than 70 beaches on a island less than 100 km2. Most are only accessible by hiking or motorcycle. The pearl of the island is Kolona Beach with its lovely half-moon shape and some of the best sand in the Cyclades.
Kythnos is a Greek island and municipality in the Western Cyclades between Kea and Serifos. It is 56 nautical miles from the harbor of Piraeus. Kythnos is 100 km² in area and has a coastline of about 100 km. Wikipedia