Recent mine disasters in the United States exposed various inadequacies and gaps in mine-safety technology. Congress responded to these tragedies by enacting the Mine Improvement and New Emergency Response Act of 2006 (MINER Act), which resulted in the most significant change to mine-safety laws in 30 years. To help meet the requirements of the MINER Act and to help eliminate future tragedies, the National Institute for Occupational Safety and Health (NIOSH) initiated aggressive research programs for developing new, and enhancing existing, mine-safety technologies. A major research emphasis addressed the lack of postaccident communications, which surfaced as a critical deficiency in some disasters. As a result, three communications approaches emerged as viable technologies - enhanced leaky-feeder, wireless-mesh, and medium-frequency systems. This paper describes the operation, application, advantages, and disadvantages of each system, as well as the challenges associated with underground wireless communications.