Explosion at Hoover Dam: Transformer catches fire at one of country's largest hydro-electric facilities as mega-drought sends reservoir to historic lows
· Explosion rocked the Hoover Dam complex on the Colorado River near Las Vegas on Tuesday
· Officials say one of the transformers at the power-generating turbine house caught fire
· Video shows flames and a massive plume of black smoke rising from a power-generating turbine house
· Frightened tourists on top of the dam were quickly moved out of the area to safety
· The blaze was extinguished in about 30 minutes, before fire units arrived from nearby Boulder City