Beneath the world's great cities, metro systems quietly shape how millions live, work, and travel. Some are century-old icons of design and engineering; others are gleaming, driverless giants that move at incredible speed. This gallery looks at 15 of the best, from London's pioneering tube to Hong Kong's clockwork MTR.
Click on to discover which networks run around the clock, which cost just a few cents, and which have the most spectacular stations.