Cal. Lab. Code § 7984

In any tunnel or underground mine classified extrahazardous a device or devices which automatically and continuously test the atmosphere for gases or vapors shall be maintained. Such device or devices shall be placed as near the face or area of operation as practical, but never more than 50 feet from such point. The division shall determine if additional monitors are necessary and where they should be located. This requirement shall apply only to tunnels or underground mines where excavation of material is by mechanical means.