A government worker's message for Elon Musk

Coal mining has always been dangerous, one of the deadliest professions in the nation. In the 20th century, more than 100,000 workers died in the U.S. mining industry.

Christopher Mark experienced that risk first-hand. "I started the mines in 1976; it was almost 50 years ago," he said. "And I worked underground altogether for about two years. And I can tell you, I was almost killed a couple times."

Mine safety has been Mark's life's work. In his 20s, he left the coalfields to earn a Ph.D., then returned as an engineer for the federal government. His efforts have helped save countless lives underground. "It was still maybe a coal miner every week that was being killed in a rock fall," he said. "In 2016, we had our first year with zero, a really tremendous accomplishment."