On the Neva River, Saint Petersburg is called the Venice of the North, because of its network of canals and bridges. The Neva freezes solid from early December to early April, but during warmer months visitors can explore the city and delta on a variety of craft from small motor launches to boats carrying up to 150 passengers. Cruising under major bascule bridges as they open is a popular after- midnight tour, particularly during the "White Nights" of early summer. Book tours in advance or at various moorings such as near Mikhailovsky Garden, Peter and Paul Fortress or Admiralty Embankment.