Flame shooting hundreds of feet from NASA's new five-segment solid rocket motor during a ground test, Sept. 8, 2011 at ATK's facility in Promontory, Utah. The ground shook. The air shook. It was so bright you couldn't look at it directly.
The test motor was 154 ft long, generated 3.6 million pounds of thrust, 22 million horsepower, and was anchored on a concrete block that goes 40 feet deep. The segment casings on this test motor flew on several shuttle missions as part of the solid rocket boosters, which were manufactured at this same site.
Bill has more photos of this test from various vantage points in his photostream, along with some iPhone video.