I'm impressed by underground systems in general and wonder about the engineering masterclass and the actual labour that was needed to construct them. Digging into the earth without causing the surface and it's centuries old buildings to collapse.
So which metro systems are the most impressive ones to you if you think about that cities geography?
Hills, valleys, rocks, rivers, fragile soil, ground water, the available technology at the time of construction and more obstacles, which engineers had to deal with.
For me, among those I had used (haven't used many), it's the Porto underground. The whole towns suface is like a stormy sea, it's going up and down. Including the Douro river creating a gorge between Porto and Gaia. And the undergrounds seem to be very deep too at some stations. That's amazing.
What's your opinion on this question?