Saint Isaac's Cathedral or Isaakievskiy Sobor (Исаа́киевский Собо́р) in Saint Petersburg, Russia is the largest Russian Orthodox cathedral (sobor) in the city. It is the largest orthodox basilica and the fourth largest cathedral in the world. It is dedicated to Saint Isaac of Dalmatia, a patron saint of Peter the Great.

The cathedral took 40 years to construct, under Montferrand's direction, from 1818 to 1858. To secure the construction, the cathedral's foundation was strengthened by driving 25 000 piles into the fenland of Saint Petersburg. Innovative methods were created to erect the giant columns of the portico. The construction costs of the cathedral made an incredible amount of 1 000 000 gold rubles. Under the Soviet government, the building was stripped of religious trappings. In 1931, it was turned into the Museum of the History of Religion and Atheism, the dove sculpture was removed, and replaced by a Foucault pendulum.

DOME The cathedral's main dome rises 101.5 metres (333 ft) and is plated with pure gold. The dome is decorated with twelve statues of angels by Josef Hermann. These angels were likely the first large sculptures produced by the then novel process of electrotyping, which was an alternative to traditional bronze casting of sculptures. Montferrand's design of the dome is based on a supporting cast iron structure. It was the third historical instance of cast iron cupola after the Leaning Tower of Nevyansk (1732) and Mainz Cathedral (1826)

INTERIOR The cathedral's bronze doors, covered in reliefs by Ivan Vitali, are patterned after the celebrated doors of the Battistero di San Giovanni in Florence, designed by Lorenzo Ghiberti. Suspended underneath the peak of the dome is a sculpted white dove representing the Holy Spirit. Internal features such as columns, pilasters, floor, and statue of Montferrand are composed of multicolored granites and marbles gathered from all parts of Russia. The iconostasis is framed by eight columns of semiprecious stone: six of malachite and two smaller ones of lazurite. The four pediments are also richly sculpted.