AP Explains: What's behind Venezuela's historic blackout?

BOGOTA, Colombia (AP) — A massive blackout that left millions of Venezuelans without power has triggered one persistent question: How could a country with the world's largest proven oil reserves go dark?

The answer involves poor decision-making, rampant corruption, and the steady flight of electrical engineers, according to experts, though the government blames sabotage.

Here's a look at what led up to the historic blackout and why it has taken so long to restore power.

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HOW DOES VENEZUELA'S POWER GRID WORK?

Venezuela's power grid relies heavily on the Guri Dam, a giant hydroelectric power station that was inaugurated in the late 1960s. While the engineers who planned Venezuela's current power grid imagined the Guri Dam would supply about 60 percent of the country's electricity, today it is believed to supply a whopping 80 percent. The remaining 20 percent comes predominately from thermoelectric power stations.

How much power Guri can actually supply and the level of Venezuela's current demand are not fully known because officials have not released that data in years. But engineers who closely examine Venezuela's power grid estimate demand has actually declined to around 10,500 megawatts — about the same levels as in 1999 — as industries have shut down and millions leave the nation.

Some engineers speculate Venezuela has even less supply than it did 20 years ago largely as a result of poor maintenance of existing plants.

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WHAT WERE THE BIG CONCERNS BEFORE THE BLACKOUT?

For years engineers have been warning that Venezuela was sliding toward a prolonged blackout.

Critics say the Guri Dam on the Caroni River basin has long needed updates and become vulnerable during periods of drought.

The late President Hugo Chavez tried to remedy the country's...