anZandt is a soft-spoken, engaging woman of 50 with round features and silver-blond hair that falls to her shoulders. She grew up a tomboy among the rolling wheat and onion fields of Walla Walla, in eastern Washington. She was fond of playing with Erector sets, and derived great joy from taking apart and putting back together household gadgets like electric clocks and radios, to see how they worked. An enthusiasm for both math and machines eventually led her to major in electrical engineering at the University of Washington in Seattle, where she was one of only two women in a class of more than 500 engineers. In spite of the overwhelmingly male milieu, she didn't feel terribly out of place. This is a woman who rides a Suzuki Bandit 1200 -- a powerful rocket on two wheels. "Sometimes," she laughs, "I like to ride it to work and shock the guards."

VanZandt brings an almost childlike sense of wonder and delight to her job as chief grid operator for the BPA. "Yeah, I'm a geek," she says with a grin. Looking out a window near her office in the Dittmer Control Center, a sixties-style concrete edifice in Vancouver, she gestures at the towers that carry high-voltage lines to a substation 100 yards away. "It's like a big Erector set. Isn't it cool?"

We walk through a broad, red-carpeted corridor in the Dittmer building and pause to examine a display that sits on the floor. It looks like modern sculpture, but it's part of a transmission tower -- a single support arm made of rigid gray insulating rubber, about five feet tall. A metal hand at one end of the arm grasps a three-foot-long sample of high-voltage power line. It's almost as thick as my wrist. "That's ACSR cable -- aluminum conductor, steel reinforced," she explains. "We wouldn't want to be this close if it were a real, energized power line. If you get within six feet of a high-voltage line at 500 kilovolts, it will flash over to you. It can be deadly business, working around this stuff." I step back, and we keep walking.

We go down some stairs and across a hall, where VanZandt unlocks the door of a small auditorium, a room with a concrete floor and walls and banked rows of comfortably padded seats. She's going to show me a PowerPoint presentation that explains the events that led to the blackout of August 14, 2003. But first, she gives me a quick lesson on how the grid works.

The three interconnects that make up the North American electric power system, she explains, are linked in a few places, but for the most part they are electrically independent. Each interconnect is a big network of wires that connect the major parts of the system with each other. At one end there are power plants. Most plants have multiple generators, which are rotors with magnets that spin inside coils of copper. This mechanical rotation generates alternating current (AC) -- a flow of electricity that changes magnetic poles at a particular frequency. In North America, that frequency is set at a standard 60 cycles per second, or 60 hertz (Hz). A generator can be made to rotate in any of several ways: by water flowing through the penstock of a hydro dam, by wind blowing an impeller, by steam created from heating water with a coal furnace or nuclear reactors, or by huge internal-combustion engines that burn diesel fuel or natural gas.

Electric current is like water in the pipes of a plumbing system; voltage is akin to the pressure that pushes the water through the pipes. The big electrical "pipes" that originate at power plants are transmission lines. Mounted on tall steel towers that march across the landscape, these lines carry gushers of current over long distances at high voltages, from 230,000 volts (230 kilovolts, or 230kV) up to a million volts. A network of 500kV transmission lines forms the backbone of the grid in the Pacific Northwest.

These electric superhighways deliver power in bulk from generators to substations, which are scattered throughout areas of high demand. Substations are the system's exit ramps. They contain transformers that step down the voltage to "sub-grid" levels as well as industrial-size switches and circuit breakers that can shut down lines if they become overloaded. Substations feed into a network of smaller wires that deliver power to its ultimate users. These smaller wires, usually suspended on wooden poles, are known as the distribution system.

Electricity is the most ephemeral of commodities. It can't be stored economically in large quantities, so it's consumed the moment it's created. Moving at close to the speed of light, electric current zips along the path of least resistance from a generator, through the transmission and distribution network, to the blender and coffeemaker on your kitchen counter. Engineers like VanZandt call anything that consumes electricity a load.

The main challenge in running a power system is balancing supply with demand -- matching generating capacity to load. Demand for electricity changes constantly, although it tends to follow fairly predictable patterns from hour to hour, day to day, and season to season. Working from demand predictions, engineers schedule power plants to increase or decrease their output by activating or idling individual generators. Human dispatchers working in control rooms can call for more or less power generation as needed to maintain balance. If there's more power flooding onto the grid than is being consumed -- or if a series of downed power lines severs the ties between a generating plant and a major load center, such as Cleveland on a steaming-hot summer day -- then bad things start to happen. Voltages drop and current starts sloshing around the grid like oil in a supertanker. Generators, which are designed to rotate in lockstep with all the other generators on the interconnect to produce a steady 60 Hz, can get out of sync. They speed up or slow down, causing vibrations that will, if unchecked, damage turbine blades, rotors, and other equipment. To prevent costly damage, relay sensors will shut down lines, generators, and entire power plants if things get hairy on the grid. And that's when the lights go off.