Why do many dense cities build subway systems underground?

Answer: Surface land is too scarce and valuable

Surface land is too scarce and valuable ✓ — Correct! In dense cities, surface space is extremely limited and expensive—filled with buildings, roads, and pedestrians. Underground metros don't compete for this space. They can run under existing streets and buildings without disrupting surface activities. Construction is more expensive underground, but in dense cities, the cost of acquiring surface land and the disruption to city life would be far greater.

Underground is cheaper to build — Wrong. Underground is actually much more expensive to build than surface rail—requires tunneling, reinforcement, ventilation, drainage, and emergency systems. Subways go underground despite the cost because dense cities have no surface space available. Surface land is too valuable and disruption would be enormous. In less dense areas or suburbs, metros often run on surface or elevated tracks because it's cheaper.

Underground is always cooler — Wrong. Underground is actually often hotter, especially in deep tunnels (geothermal heat, train friction, crowding). Subways go underground primarily because dense cities lack surface space for rail lines and stations. Underground doesn't disrupt existing streets, buildings, and pedestrian flow. Some systems (like London) have expensive cooling systems because underground gets too hot, not cool.