Where is the DEEPEST metro in the world? In St. Petersburg!
The Northern Capital is the world record holder for its subway system’s depth: it is on average 60 meters deep!
The subway stations are built at such a depth, because the upper ground layers under the city are moist and unreliable. The hydrological network of St. Petersburg includes about 100 rivers and canals with a total length of almost 300 kilometers. There are also underground palaeovalleys – traces of a not active river with unstable ground and a large amount of groundwater.
One can only build tunnels through places with a thick sedimentary rock – Cambrian clay. But, to reach it, one often has to freeze quicksands – sands saturated with water. Only then can you begin drilling a hole.
St. Petersburg also has the deepest subway station in Russia at 86 meters deep – Admiralteyskaya Station. And the longest escalator in the world leads to the platform (137.4 m). But, even that is not enough to reach the platforms. So, another, shorter escalator had to be installed.
