One of the largest cathedrals in the world of any denomination, St Isaac’s gargantuan size is certainly staggering to behold; its enormous gold-plated dome is visible from almost anywhere in Saint Petersburg. As it took 40 years to build, grander and more elaborate designs kept getting added to the original plans: over 100 massive columns were erected alongside several other smaller domes. Inside is even more impressive as beautiful reliefs, mosaics, and iconostasis cover every imaginable surface. Although it still holds services, St Isaac’s Cathedral was turned into a museum in 1931 by the Soviet government and remains so to this day.