Visual warning signals associated with rockfalls in deep-level gold mining are embedded in a misleading and competing background. Due to the relatively high incidence of rockfall accidents and the relatively high involvement of novice mine workers in accidents, the ability of mine workers to discriminate visually between dangerous and safe rock conditions was examined. Stimuli were presented to participants with the aid of stereoscopic slides and responses were analysed in terms of the Theory of Signal Detection. The results confirmed the contention that underground experience as well as skills training improves significantly the ability of mine workers to discriminate visually between dangerous and safe rock.