Which metro system is considered the deepest in the world on average, due to the city's soft, waterlogged soil?

The Saint Petersburg Metro is considered the world's deepest metro system on average, largely because the city was built on soft, waterlogged ground.

Saint Petersburg sits on a low-lying delta beside the Neva River, with marshy deposits, a high water table, and difficult soil conditions. Engineers therefore had to drive many tunnels and stations deep into more stable geological layers. The system's average station depth is commonly given at about 60 metres, although the exact figure varies by measurement method and by whether tunnels or stations are counted.

The network opened in 1955 and uses deep-level construction extensively. Admiralteyskaya, the system's deepest station, is about 86 metres underground. Long escalator rides are a familiar feature, especially in central stations, because passengers must descend from street-level vestibules to deeply buried platforms.

This distinction is easy to confuse with individual-station records. Kyiv's Arsenalna and stations in Pyongyang or Moscow are often cited among the world's deepest single stations. Saint Petersburg's claim concerns the system-wide average, not necessarily the single deepest platform.