Ukraine is a sovereign state in Eastern Europe, The territory of modern Ukraine has been inhabited since 32,000 BC. During the Middle Ages, the area was a key centre of East Slavic culture, with the powerful state of Kievan Rus' forming the basis of Ukrainian identity. During the 20th century three periods of independence occurred. The first of these periods occurred briefly near the end of World War I and the second occurred, also briefly, during World War II. However, both of these first two earlier periods would eventually see Ukraine's territories consolidated back into a Soviet republic within the USSR. The third period of independence began in 1991. Ukraine has a mostly temperate climate, with the exception of the southern coast of Crimea which has a subtropical climate. Ukraine has numerous tourist attractions: mountain ranges suitable for skiing, hiking and fishing: the Black Sea coastline as a popular summer destination; nature reserves of different ecosystems; churches, castle ruins and other architectural and park landmarks; various outdoor activity points. Kiev, Lviv, Odessa and Kamyanets-Podilskyi are Ukraine's principal tourist centres each offering many historical landmarks as well as formidable hospitality infrastructure.