Saint Isaac´s Cathedral is the largest cathedral in Saint Petersburg. The Cathedral was constructed by French-born architect Auguste de Montferrand, it took many years to complete the construction (1818 to 1858). Under the Soviet government, the building was abandoned, then turned into a museum of atheism. Nowadays worship activity has resumed in the cathedral, but only in the left-hand side chapel, and in the main body of the cathedral on feast days only.

Sources, related links:
Wikipedia - Saint Isaac´s Cathedral
Saint Isaac´s Cathedral - official web site