The invention provides a rock wall closure detection apparatus which includes an elongate member, variably extensible between a first wall and a second wall Of an underground excavation which comprises of a first and a second segment which are resiliently axially compressible relatively to one another when the waits close and an indicator module which produces a warning signal when actuated, attached to the first segment and an actuator attached to the second segment wherein on compression of the member, the actuator moves relatively to the module to actuate the module to produce the warning signal.