Megacities like Tehran are silently sinking

Tehran has been experiencing a geological phenomenon that at first seems harmless and incoherent: be it local floods, a crack in the house wall or uneven paving stones. In the metropolitan area of Tehran, the city sinks as low as 25 centimeters per year in certain places. The geological phenomenon is called subsidence.

Why it matters. Some people attribute a hole in the asphalt to amateurish road construction, drought, but the capital, which is home to several millions of people, has one of the highest rates of subsidence in the world.