Final answer:
St. Petersburg faced the significant challenge of being built on a swamp, leading to frequent flooding which required extensive engineering to overcome.
Step-by-step explanation:
The major problem that plagued the city of St. Petersburg was that it was built on a swamp that flooded often. This created significant challenges for the city, as swamps are not typically ideal locations for the development of major urban centers due to their unstable ground and tendency to flood. Overcoming the environmental difficulties of swampy terrain required ingenious engineering and substantial effort to establish a stable foundation for St. Petersburg and prevent flooding, which would have been a recurrent problem for inhabitants. Despite these challenges, St. Petersburg managed to develop into Russia's second-largest city, a cultural center, a major port on the Baltic Sea, and an important location for shipbuilding, oil and gas trade, and finance.