Nisyros (also transliterated Nissiros) is a volcanic Greek island and municipality located in the Aegean Sea. It is part of the Dodecanese group of islands, situated between the islands of Kos and Tilos. Its shape is approximately round, with a diameter of about 8 km (5 mi), and an area of 41.6 km2 (16.062 sq mi). Several other islets are found in the direct vicinity of Nisyros, the largest of which is Gyali. The Municipality of Nisyros includes Gyalí (pop. 10) as well as uninhabited Pacheiá, Pergoússa, Kandelioússa, Ágios Antónios, and Stroggýli. It has a total land area of 50.055 km2 (19.326 sq mi) and a total population of 948 inhabitants. The island was also called Italian: Nisiro.
The island has a mountainous interior and in its center several craters are found. Its coasts are generally rocky or pebbled, but there are also a few sandy beaches (mainly in the northeastern part). The volcano is currently active (but not erupting), and fumaroles are found at the craters. The latest eruptive activity was a steam explosion in 1888, after small ash eruptions in 1871 and 1873 and earthquakes are not infrequent. A period of seismic unrest in 1996-1997 led an international team of scientist to initiate monitoring of the volcanic unrest in the European Union sponsored Geowarn project. The entire volcanic complex includes the seafloor between Nisyros and Kos, the island of Gyali, and a part of Kos island.
A traditional product of Nisyros is soumada, a non-alcoholic almond-flavoured drink. The patron saint of the island is Saint Nikitas. Many Orthodox Christian churches are found on the island, as well as four monasteries which are not inhabited by monks today, although various celebrations take place in them. The largest monastery is the one of Panagia Spiliani (Blessed Virgin Mary of the cave) at Mandraki. It is built beside the medieval castle erected by the Knights Hospitaller who conquered the island in 1315.