Mo. Rev. Stat. § 293.540

After a reasonable time has been allowed for drifting and development operations, the director may require that a noncoal mine operating through either a shaft, slope, or drift, have not less than two adequate and accessible outlets as a condition precedent to employment of any person in the mine. In mines opened after the effective date of this chapter, all vertical shafts shall be located not less than one hundred fifty feet apart.