It’s called Day Zero — the day when dams drop below critical levels and Cape Town officials shut off water to most homes and businesses in South Africa’s second-largest city. The metropolitan region of nearly 4 million people faces an immense challenge to prevent this scenario from happening: cut water use to 50 liters per person per day. Even conservation of that magnitude might not be enough to avert Day Zero. How the city and its residents respond to the drought crisis is a case study for this era of perilous environmental change.