Actavis (NYSE:ACT) sold the second-biggest corporate-bond offering in history yesterday, with a $21B deal fueled by investors' desire for income-generating investments when yields on government debt remain low. The giant sale comes as companies this year sell debt at the fastest pace on record, extending a multi-year boom that has financed a flurry of deal making and stock buybacks. Actavis plans to use the money for its $66B acquisition of Botox maker Allergan (NYSE:AGN).