100 meters for things built for humans. 1000 meters for mines.
image edited by me.
Subways can be as deep as the foundations of these huge buildings.
But if you think these stations are rather like descending to hell,
they are nothing compared to the Arsenalna underground station in Kiev
(Ukraine). Its escalator will take you down to the centre of the earth
in order to get on one of its trains, which run more than 100 metres
below ground. It is the deepest underground station in the world.
These structures built for humans are puny in comparison to mines. Mines are more than ten times as deep, and really big.
source for image
At 2,300 feet, the mine is the deepest in North America. That's deep
enough, Wilczynski notes, to nearly stack two Empire State buildings,
which has a roof height of 1,250 feet. The mine shaft is at Portland
Point, on the eastern shore near the south end of Cayuga Lake, and the
mine itself stretches north along the lake to a mile past Taughannock
Point, the company's website says.
The mine stretches more than seven miles underground, tapping into a
vast salt deposit that stretches from Pennsylvania to Ontario.
The deepest mine in the world is the Mpoeneng gold mine, which is working on becoming 4300 feet deep.
In sum, it looks like the deepest structures purpose built to house and move humans are 100m / 300 feet deep. Mines, though, are built because of different practicalities - miners are retreiving things that are underground and so go much deeper to where those things are. The farther down you go the trickier ventilation gets, but there is no reason why a city cannot use old mines that happen to be convenient for expansion - both Paris and Kansas City do.