Kythnos is one of the few islands characterised by the unspoiled beauty of the Cycladic nature and also distinguished by a rich and authentic physiognomy that resists mass tourism. It is the second island in the chain of the Western Cyclades located between the islands of Tzia and Serifos. The island, even to this day, is often referred to the medieval name Thermia, due to the famous thermal baths located in the area of Loutra. The size of Kythnos reaches 99 sq.km. and the length of the coastline is about 104 km. The shape of the island is relatively long, 21 km long and 11 km wide and its direction is north to south. The landscape is typically Cycladic with terraces, hills, small flats, streams, cliffs and divided coasts that create dozens of little coves and beaches. Kythnos hosts the astonishing number of 72 sandy beaches. The terrain has a hilly relief with a typical backbone, from north to south, interrupted by small plains. The highest hill is the Kakovoulo mountain at 355 meters to the north, followed by Prophitis Helias (326 m.), Merovigli (318 m.), Larni (315 m.), Atheras (291 m.), Kalogeritsa (267 m.) and Agioupes (263 m.). Geologically the island is composed of slates with few areas of limestone and volcanic rocks. The most important seasonal streams are Vathirema, Episkopi, Galliou, Kourasmenos, Ardikopos, Petraina, Pentafilia and Andrianos. Throughout the island there are a lot of small springs that keep water even in the summer. The trademark of Kythnos is the enchanting 220 meter long sand strip that creates two beaches and connects the island with the islet of Agios Loukas, a geological formation called tombolo. A very interesting sight of the island is the big and accessible Katafiki cave, located in the village of Dryopida. The lacy shoreline creates a wonderful sea bed that magnetizes dozens of divers every year. Kythnos is a unique destination for exploration, as it has not been extensively studied. The long coastline, the vast brushwoods, the countless paths and the absence of any significant disturbance make Kythnos an ideal habitat for many species of Cycladic flora and fauna.
The island is characterised by the mediterranean vegetation, while the small streams between the hills create spots of intense vegetation, mainly from reeds, oleanders and brambles. Few places hold big valonia oaks, remnants of the ancient forests that covered the island, a botanical fact that is also confirmed by the ancient name of the island that has survived to this day, Dryopis (full of oaks). Other trees are tamarisks, seashore junipers, phoenicean junipers, carobs, wild olives, almond-leaved pears and mediterranean cypresses. Lentiscs, Jerusalem sages, thymes, thorny burnets, asphodels, common sages and various shrubs cover the entire island. The most important plants of Kythnos are the endemic Fritillaria obliqua subsp. tuntasia and Silene cythnia that took its name from the island. Other important plants are Campanula reiseri, Anthyllis splendens, Centaurea laconica subsp. lineariloba, Muscari pulchellum subsp. clepsidroides, Bonannia graeca, Hymenonema graecum, Nigella doerfleri, Nigella degenii subsp. degenii, Crocus tournefortii, Ferulago humilis, Carthamus leucocaulos, Solenopsis laurentia, Dianthus fruticosus subsp. fruticosus, Dianthus tripunctatus, Lotus palustris and Limoniastrum monopetalum. Some interesting plants of Kythnos are Calystegia soldanella, Crocus laevigatus, Crocus cartwrightianus, Pancratium maritimum, Centaurea spinosa, Echinops graecus, Anchusa aegyptiaca, Limonium aegaeum, Erygnium maritimum, Bellium minutum, Opopanax hispidus, Pimpinella cretica, Onopordum tauricum, Arenaria muralis, Ononis diffusa, Filago aegaea subsp. aegaea, Helichrysum italicum, Alyssum umbellatum, Delphinium staphisagria, Malcolmia flexuosa subsp. naxensis, Matthiola incana subsp. incana, Silene pentelica, Mandragora officinarum and Velezia quadridentata. The orchids of Kythnos are Anacamptis boryi, Anacamptis collina, Anacamptis coriophora subsp. fragrans, Anacamptis papilionacea subsp. aegaea, Anacamptis sancta, Orchis quadripunctata, Serapias bergonii, Serapias lingua, Serapias orientalis, Serapias parviflora, Ophrys bombyliflora, Ophrys phryganae, Ophrys cretica subsp. cretica, Ophrys ferrum-equinum, Ophrys iricolor, Ophrys sancti-isidorii and Ophrys tenthredinifera.
Kythnos is an important migratory station for many species of birds. The most important of them is the Eleonora’s falcon that arrives in the spring and makes its nests on the cliffs of the island. Kythnos is known for the large numbers of chukar partridges that unfortunately have been depleted due to intensive hunting.The birds of prey in Kythnos are Bonelli’s eagles, long-legged buzzards, common buzzard and kestrels, while in the migratory periods arrive lesser kestrels, hen harriers and pallid harriers. Little owl are abundant here, while the nocturnal predators include scops owls and barn owls. The reports of eagle owls on the island are probably referred to long-eared owls in migration. The seabirds include yellow-legged gull, Scopoli’s shearwaters, mediterranean shearwaters and shags. The seasonal wetlands attract little egrets, grey herons, purple herons, squacco herons, moorhens, black-winged stilts, kentish plovers, little ringed plovers, sandpipers, greenshanks, redshanks and marsh sandpipers. Other birds that can be observed here are quails, turtle doves, cuckoos, nightjars, swifts, alpine swifts, pallid swifts, crag martins, house martins, swallows, hoopoes, kingfishers, bee-eaters, crested larks, meadow pipits, yellow wagtails, white wagtails, black redstarts, northern wheatears, black-eared wheatears, stonechats, blue rock thrushes, blackcaps, lesser whitethroats, sardinian warblers, subalpine warblers, Rüppell’s warblers, willow warblers, chiffchaffs, spotted flycatchers, red-backed shrikes, woodchat shrikes, ravens, hooded crows, linnets, goldfinches, Cretzschmar’s buntings and corn buntings.
There are only two amphibians living in Kythnos, the balkan frog and the green toad. The island during the antiquity was known as Ophiousa (full of snakes), something that reflects the rich erpetofauna of Kythnos. The island is home to the endemic subspecies of the Erhard’s wall lizard (Podarcis erhardii subsp. thermiensis) and balkan green lizard (Lacerta trilineata subsp. hansschweizeri). Other reptiles are balkan pond turtles, Kotschy’s geckos, mediterranean house geckos, snake-eyed skinks, ocellated skinks, large whip snakes, levant montpellier snakes, grass snakes, Dahl’s whip snakes and leopard snakes. The appearances of loggerhead sea turtles around the island are very common. From the mammals on Kythnos live beech martens, eastern hedgehogs, small rodents and bats. The hares that were almost everywhere in the island have been declined. The entire coastline of Kythnos is full of sea caves, thus preserving a small population of mediterranean monk seals. In the inland caves there are a lot of rare invertebrates, of which one of them, the millipede Cordioniscus kithnosi, lives only in Kythnos. Other rare and endemic invertebrates on the island are the slug Deroceras keaensis, the beetle Dendarus kochi and the isopod Ligidium cycladicum. Among many species of butterflies, giant peacock moths (Saturnia pyri) are fairly abundant here.