Aubrey McIntosh has taken up mining in his spare time, and he's finding it hard and hot—even if it's prospecting for a virtual currency and a computer is doing all of the work.

Mr. McIntosh, a semiretired chemistry professor in Morris, Minn., is among the growing ranks of enthusiasts who use powerful computers to "mine," in insider parlance, "bitcoins," an unregulated digital currency.

Mr....