30 CFR § 57.4560 - Mine entrances.
For at least 200 feet inside the mine portal or collar timber used for ground support in intake openings and in exhaust openings that are designated as escapeways shall be -
(a) Provided with a fire suppression system, other than fire extinguishers and water hoses, capable of controlling a fire in its early stages; or
(b) Covered with shotcrete, gunite, or other material with equivalent fire protection characteristics; or
(c) Coated with fire-retardant paint or other material to reduce its flame spread rating to 25 or less and maintained in that condition.