A quick primer on how electricity works: First, power plants create it, mostly by burning fuel (or smashing atoms) that heats water to make steam that spins a turbine. (Hydroelectric generators harness the flow of water to spin turbines directly.) Those turbines move a generator, which produces electricity from the resulting kinetic energy. Plants then use transformers to step up the voltage of generated electricity and send it down high-voltage lines, which lose less energy in transit. Once it reaches its destination, other transformers step the voltage down to deliver it to substations, and eventually directly to customers.

Saturday’s blackout was most likely caused by a disabled transformer at an area substation. There are at least 50 of those in New York City, which are fed in turn by at least 24, higher-voltage transmission substations. When it comes to power, New York is unusual because of the city’s age and the density of its population, both residential and commercial. That produces different risks and consequences.

In Atlanta, where I live, storms often down trees, which take out aboveground power lines. In the West, where wildfires are becoming more common, flames frequently dismantle power infrastructure (sometimes the power lines themselves cause the fires). But across the whole of New York City—not just Manhattan—more than 80 percent of both customers and the electrical load are serviced by underground distribution from area substations. That makes smaller problems less frequent, but bigger issues more severe.

When a transformer goes down in a populous place like Manhattan, it has a greater impact than it would on Long Island, say, or in Westchester County, where density is lower. The amount of power that central Manhattan uses on a regular basis also contributes to that impact. Times Square, the theater district, hundreds of skyscrapers—it’s a substantial load. In New York’s case, supplying that load is not usually the problem. Generating facilities can be located near or far away from where their power is used, and New York City draws power from a couple dozen plants. Some of it is imported from upstate.

But much of New York’s power is still generated locally, in large part at plants along the waterfront of Queens. Those plants are older, and more susceptible to disruption from local calamities, especially severe weather. When peak demand surges—most common during heat waves, such as the ones that struck the region in 2006 and 2011—the older, less efficient generating stations have a harder time keeping up, and brownouts or blackouts become more likely.

Superstorms can also disrupt Manhattan’s delivery infrastructure, despite the fact that it’s underground. In 2011, Hurricane Irene threatened to flood traffic and subway tunnels, also putting underground delivery at risk. The next year, Hurricane Sandy disrupted a third of the city’s electrical capacity. Flooding shut down five transmission substations. Other infrastructure was affected too, including natural gas and steam services (the latter provide heat and hot water, crucial during winter and for emergency facilities such as hospitals).