MSHA - Code of Federal Regulations - 30 CFR 57.4360
30 CFR § 57.4360
Underground alarm systems.
(a) Fire alarm systems capable of promptly warning every person
underground, except as provided in paragraph (b), shall be provided and
maintained in operating condition.
(b) If persons are assigned to work areas beyond the warning
capabilities of the system, provisions shall be made to alert them in a
manner to provide for their safe evacuation in the event of a fire.