Ancient landslides at two locations in Ventura County, California, USA, that have morphology consistent with thousands of years of stability were reactivated in 1988 and 1992–1994. The timing of reactivation was assisted by the burial or destruction of oil wells that penetrated through the landslides and had operated continuously since at least 1948. Slope movement at Grimes Canyon began during a period of lower-than-normal rainfall (compared to average precipitation since 1948), which suggests that irrigation triggered instability. At South Mountain, landsliding initiation overlapped a period of intense rainfall; however, the influence of irrigation, which may have contributed antecedent groundwater conditions conducive to slope failure, may not be ruled out. These case studies illustrate the complexity of identifying landslide triggering mechanisms, and they show that triggering of deep-seated landslides may be aided by climatic conditions or irrigation.