Saint Petersburg is built on what originally were more than 100 islands created by a maze of rivers, creeks, canals, gulfs, lakes and ponds and other bodies of water that flow into the Baltic Sea at the mouth of the Neva river. Renown for its numerous bridges over canals and rivers of various sizes, styles and constructions, built at different periods. St. Petersburg is often called the "Venice of the North" or the city of palaces. It has many cathedrals and buildings of baroque and neoclassical construction. The largest cathedral - St Isaac's Cathedral - houses the biggest gold-plated dome in the world.