Roger Slack paced in the control tower at Eldora Speedway, watching the cars race past on the track far below. Every few moments he said something into the small radio he clutched in his hand, glancing at the lap counter and watching the flagman. As the race winner flashed across the finish line Slack’s eyes widened as two propane burners belched flame at the precise moment, while several massive fireballs mushroomed off the terrain just beyond the backstretch. An instant later the shock wave MORE