Odesa, Ukraine

Odesa sits on the Black Sea with roughly one million residents, making it Ukraine's third-largest city and its main seaport. Catherine the Great founded the city in 1794 on the site of the ancient Greek colony of Histria. The Potemkin Steps, completed in 1841, drop 27 meters across 192 steps that widen from 12.5 to 21.7 meters to create an optical illusion when viewed from above. Beneath the streets, the 1820 Odesa Catacombs extend about 2,500 kilometers, forming the largest urban catacomb network in the world. The 1810 Opera House by the Viennese architects Fellner and Helmer remains one of the oldest such venues in eastern Europe, and Alexander Pushkin lived here in 1823 and 1824.