The revisionist scholarship on colonial science assumes its inherent rationality. The example of water divining in southern Africa, however, suggests that the irrational was as much a feature of western as indigenous knowledge systems. The state-led opening of an underground water frontier in the arid (Karoo) interior of the Cape Colony in the two decades after 1890 brought this issue into sharp focus. State water boring was guided by a combination of geological and engineering science, but encountered sustained resistance from settler farmers who preferred the word of their water diviners over the official experts in deciding where to bore. After failing to suppress the practice, the colonial state belatedly promoted and adopted it after water-boring was privatized in the mid-1900s. A detailed analysis of the wealth of correspondence on the subject in the department of agriculture journal after 1905 reveals both a sustained attempt by supporters to rationalize divining and a reticence on the part of skeptics to submit to a definitive empirical test. The debate over water divining suggests that colonial ideologies of agricultural improvement were more eclectic and irrational than crude dichotomies opposing western rationality to native superstition allow. In short, the other was within as well as without.