Failure of natural slopes may result in considerable damage to both human health and real estate properties. The conditions “necessary” for a slope to fail have been investigated thoroughly in the past. Additional work is still necessary regarding the causes that trigger a slope to move and that may in the long run even be helpful to predict the failure of a slope. We conducted experiments to investigate the influence of various groundwater conditions on slope stability. Our results showed that the position of the groundwater table and especially a variation in pore water pressure greatly affect the slope stability under loaded conditions. Moreover the results showed that slope failures are preceded by an abrupt decrease in pore water pressure. We suggest that a monitoring of the fluctuation of pore water pressure in a real slope may give an indication of its inherent stability.