We used to fly a lot of low level in the B-52. It was thought to be the only way to avoid the radar sites protecting the USSR so we practiced it a lot. One balmy afternoon we we flying a low level route in the midwest. It was an upgrade ride and the pilot and the radar nav (yours truly.) We were getting evalutated for advancement to instructor. And evaluator pilot was sitting in the jumpseat between the pilots and his counter part nav was sitting downstairs with me and the nav evaluating me. We entered the low level training route flying at 400 feet above the terrain and about 300 miles per hour so we were all on our toes. Then the pilot calmly reported, "We have a fire warning light on engine number 3. Let's climb." I rogered him, checked my chart and told him to climb to 4500 feet. We started to climb when he reported, "Okay, now we have a fire light on number 4. We're shutting them both down. Nav, give us a heading back to base. Radar (that's me. There are 2 navigators on a B-52; the navigator who plots our position and the Radar Nav, or RN, who updates the system and drops the bombs.) report that we're aborting the route and get us clearance to the homedrome." I contacted the FAA controller and told him we were climbing out of low level and wanted to go direct back to our base in Texas. He asked if we were declaring an emergency and the instructor pilot chimed in on the radio that we were definitely declaring. Declaring an emergency would give us preferencial treatment by all the air traffic control agencies. Now a fire light on an engine in a BUFF ( ackronym for Big Ugly Fat F**ker the standard crewdog name for a B-52) is bad but not terrible. It usually goes out when the engine is shut down. And since the BUFF has 8 of them it's no big deal to shut down one...or two. But 2 fire light in two engines right next to each other is cause for tension. We got headed back to the base on a direct course and we cleaned up the airplane from training. The IP took over command of the aircraft from our pilot as he is required to do by regulation. Just as he got on the interphone he said something that just about stopped my heart. He said, "Holy crap, look at that explosion!" I grabbed the leg straps of my parachute, tightened them so tight that I cut of circulation, unstowed the trigger ring of my ejection seat and was just about to jettison the aircraft when the copilot said, "Sir, you think you should tell the crew that the explosion was on the ground?" Yep. We were flying near a gunnery range and the jerk saw a practice round go off. And he nearly got the whole crew to bailout. And he bought the beer for the next two weeks.