Officials there are trying to figure out what caused the worst U.S. mining disaster in 25 years. It happened Monday afternoon at the Upper Big Branch South Mine: a massive blast inside the coal mine that killed at least 25 people, left 4 others trapped. Rescue workers are trying to drill their way into the mine, but they need to get 1,200 feet down inside of it, and that could take a while. Not only is the process slow, it's dangerous. Crews are having to deal with potentially harmful gases. President Obama and other officials, including both of West Virginia's U.S. senators, have offered their thoughts and prayers to the families of the miners. They've also said they're determined to find out what happened and how it can be prevented in the future. One government official said, "Miners should never have to sacrifice their lives for their livelihood." This all happened inside a facility that does what's called longwall mining.